Highmark Health sustained its financial strength and stability through 2020, with excess revenue over expenses of $450 million and an operating gain of $490 million, excluding a one-time gain. While most business units performed better than prior year, enabling us to reinvest more than $750 million back in our communities, the positive performance was offset by challenges on the provider front as routine care was rescheduled as a result of the pandemic. In addition to realizing revenue of $18 billion, the organization maintained a strong balance sheet with about $11 billion in cash and investments at year end, and net assets of about $9 billion.
Highmark Health Plans
The Highmark Health Plans reported an operating gain of about $400 million for the 2020 fiscal year, excluding a one-time gain, primarily driven by effective performance in the commercial and government business and a continued focus on clinical transformation activities. Due to these favorable results, Highmark Health Plans were able to provide benefits to members through rebates and premium credits, which is in line with our mission to create a remarkable health experience.
Core health plan membership remains steady with approximately 5.6 million Highmark members through year end, with total commercial retention rates at 93%. Highmark remains the commercial market leader across the areas it serves, with a retention rate of 95% in western PA. Highmark also saw growth in its High Performing Network Products, with a 3% year-over-year membership increase.
Highmark remains the largest health insurer in Pennsylvania, West Virginia, and Delaware, and has a broad national reach. As Highmark welcomes HealthNow to its family in 2021, it expands its footprint into New York, adding more than one million members to its diverse network.
Allegheny Health Network
Following three years of continued growth, Allegheny Health Network delivered an operating loss of $136 million through December 31, a decrease of $180 million compared to 2019 performance, due to volume declines associated with the COVID-19 pandemic and pandemic-related expenses.
For 2020, patient volumes decreased year over year in several key categories, including a 9% decrease in inpatient discharges and observations, a 2% decrease in outpatient registrations, and a 7% decrease in physician visits. Patient volumes continued to recover through year end after experiencing a low point in second quarter 2020.
Highmark Health Diversified Businesses
Highmark Health's diversified businesses reported combined earnings of approximately $370 million through December 31, an improvement of more than $180 million from prior year due to solid performance in dental and stop loss segments. This performance is largely driven by deferred patient care as a result of the COVID-19 pandemic.
United Concordia Dental and our Blue-branded dental business continue to perform well, delivering an operating gain of $254 million. A stop loss business, HM Insurance Group (HMIG), reported an operating gain of $101 million.
HM Health Solutions
HM Health Solutions, an information technology services company whose platform serves more than 10 million lives across the country, reported an operating gain of approximately $14 million driven by internal transformation initiatives.