Highmark Health delivered its second-best consolidated financial performance since its formation in 2013, with an excess of revenue over expenses of $570 million, and an operating gain of $526 million, driven by strong operating performances across all of its business units. Total revenues improved over prior year by more than $500 million to nearly $18.8 billion, while Highmark Health's balance sheet continued to strengthen, with net assets of $6.7 billion, an increase of $200 million over the prior year driven by earnings, and cash and investments of $8 billion as of year-end.
Highmark Health Plans
Coming off record earnings in 2017, the Health Plan again delivered strong financial performance across all segments in 2018, with operating results of more than $500 million for the year ended December 31, 2018. Commercial business results benefited from a continued focus on cost management to bring value to our customers in all the markets where the Health Plan operates.
Highmark's government business which includes Medicare Advantage, ACA, Medicaid and small group segments maintained solid margins as a result of product offerings and continued focus on cost management. The cost of care for our ACA membership stabilized in 2018 through the implementation of care management programs and benefit designs that encourage the use of lower-cost, higher-quality care options.
Health Plan membership is stable with 4.5 million members in our core markets of Pennsylvania, West Virginia and Delaware. Highmark remains the commercial market share leader in western PA with a retention rate of 94 percent and membership of 2.3 million, as we continue to offer key differentiators with our client services and ancillary product offerings.
Allegheny Health Network
Allegheny Health Network sustained its positive operating performance trajectory, reporting operating income of $39 million, an improvement of $10 million over prior year. Allegheny Health Network also reported total operating revenues of $3.3 billion, a year-over-year increase of $213 million, or nearly 7 percent, and a 5-year increase of 50 percent. What makes this significant is that it occurred despite the current industry trends of market contraction and patient transition from inpatient to outpatient facilities.
Allegheny Health Network experienced a 2.4 percent increase in combined total discharges and observations and a nearly 4 percent increase in physician visits, compared to the previous year. The health system also successfully reduced emergency department visits by 2.6 percent, attributable to the success of its Same-Day Appointments program, which increased nearly 34 percent year over year to more than 207,000 appointments in 2018.
Allegheny Health Network continues to see strong, consistent growth in its ambulatory footprint, nearing 23,000 annual surgical cases in 2018 as well as market share gains driven almost exclusively through organic growth. Key investments in Allegheny Health Network in 2018 totaled nearly $350 million.
Highmark Health Diversified Businesses
Highmark Health's Diversified Businesses which include our dental, stop loss and vision companies delivered operating earnings of $159 million, an improvement of $56 million over prior year, due to strong performances in our dental and stop loss segments, reinforcing the value of Highmark Health's diversification strategy. United Concordia, our dental business, delivered operating earnings of nearly $100 million, which is comparable to prior year, and reflects a full year of servicing the TDP contract and continued growth in network leasing partnership revenues. With more than 127,600 unique dentists in its Network and 8.5 million members, United Concordia is now the fourth-largest dental company in the country, approaching nearly $1.5 billion in revenues. Highmark's stop loss business, HMIG, delivered a significant turnaround from a challenging 2017 performance, with an operating gain of $80 million in 2018, an improvement nearly $100 million over prior year, driven by actions taken in 2017 and throughout 2018 by management to ensure improved operating performance and to restore profitable margin levels. Visionworks, our vision retail and services company, continues to deliver on its turnaround plan under its new leadership team. The company reported an operating loss of $21 million, an improvement of $21 million over prior year, excluding the results of managed vision care.
HM Health Solutions
HM Health Solutions, our information technology services company, continued its stable financial performance, growing revenues to more than $900 million. HM Health Solutions completed the migration of two major Blues plans onto the platform, which now serves 10.3 million lives.