Highmark Health sustained its financial strength and stability through 2019, with an excess of revenue over expenses of $843 million and an operating gain of $241 million, driven by continued strong financial performance across all of its affiliated companies, gains from the strategic divestment of Visionworks, and strong equity market performance. In addition to realizing revenue of $18 billion, the organization maintained a strong balance sheet with $9.1 billion in cash and investments at year-end and net assets of $7.6 billion.
Highmark Health Plans
The Highmark Health Plans delivered a solid financial performance in 2019, with operating results of $360 million. A very strong performance in the national market, which experienced a retention rate of 100%, offset an expected decline in enrollment due to the Health Plans' exit from the West Virginia Medicaid market during 2019. Health Plan membership remains strong and stable with approximately 5.6 million members, while Highmark Inc. remains the commercial market leader in western Pennsylvania with a retention rate of 98%.
The Health Plans' government business delivered another strong financial performance in 2019, driven by significant enrollment increases. Group Medicare Advantage client and membership retention improved dramatically from 85% in 2019 to 97% in 2020, while the ACA book of business experienced nearly 30% growth, now totaling more than 92,000 members.
With Gateway Health Plan dual eligibles, total Senior Product enrollment is 439,000, an increase of more than 12,000 over prior year. Total Medicare Advantage enrollment (Direct Pay and Employer Group) is 238,000 as of January 2020, up 7,000 year over year.
Allegheny Health Network
Allegheny Health Network (AHN) delivered operating income of $44 million, an improvement of nearly $5 million over prior year, and has now reported positive operating income for 11 consecutive quarters, an indicator of the network's strong financial position. In 2019, AHN reported total operating revenues of $3.6 billion, a year-over-year increase of almost $300 million, or nearly 9%, and a 4-year increase of 35%, as well as net income of $98 million, an improvement of $95 million year over year. That improvement is largely attributable to the network's strong investment performance in 2019.
AHN reported approximately 124,000 total discharges and observations, an increase of nearly 3% over 2018. AHN's decline in discharges from 2018 to 2019 of nearly 4% was more than offset by the network's growth in observation stays of more than 20%. This is consistent with industry trends, as well as AHN's and Highmark Health's organizational goals of driving care into lower-cost settings when clinically appropriate.
Oncology visits grew by nearly 9%, a significant shift that reflects the company's commitment to providing high-quality choices for cancer care in the region. Emergency Department visits grew slightly year over year, to 280,000, as did outpatient volume, which continued to grow in every segment in 2019. Outpatient registrations increased by more than 3%; and births at AHN Hospitals totaled 8,139, an increase of more than 5% over 2018.
Hospital-based surgical cases grew by about 1% in 2019 (to 84,428), while ambulatory surgery center volume grew by nearly 9% (to 24,846 cases) and physician visits grew by more than 10%.
*Operating income does not reflect new pension accounting guidance.
Highmark Health Diversified Businesses
Highmark Health's diversified businesses which include dental and stop loss companies and the vision business for a portion of the year delivered combined operating earnings of $167 million, an improvement of approximately $11 million over prior year, due to strong performances in the dental and stop loss segments, reinforcing the value of Highmark Health's diversification strategy.
The dental business, including United Concordia, delivered operating earnings of $116 million, an improvement of $15 million over prior year due to strong performance in all major business segments.
Highmark's stop loss business, HM Insurance Group, delivered strong results in 2019, with an operating gain of $63 million and revenues of $720 million, an improvement of $14 million over prior year. In 2019, Highmark completed the strategic divestment of the Visionworks business as we continue to review our portfolio of businesses, delivering a significant one-time financial gain. For the year, the Vision business reported an operating loss of $12 million, an improvement of almost $13 million over the prior year.
HM Health Solutions
HM Health Solutions, the information technology services company, continued its stable financial performance, with an operating gain of approximately $4 million, while further investing in the platform which serves more than 10 million lives.