UnitedHealthcare, which includes UnitedHealthcare Employer & Individual, UnitedHealthcare Medicare & Retirement, UnitedHealthcare Community & State and UnitedHealthcare Global;
Our businesses participate in the United States, South America and certain other international health markets. In the United States, health care spending has grown consistently for many years and comprises 18% of gross domestic product (GDP). We expect overall spending on health care to continue to grow in the future, due to inflation, medical technology and pharmaceutical advancement, regulatory requirements, demographic trends in the population and national interest in health and well-being. The rate of market growth may be affected by a variety of factors, including macro-economic conditions and regulatory changes, which have impacted and could further impact our results of operations.
We are increasingly rewarding care providers for delivering improvements in quality and cost-efficiency. As of December 31, 2018, we served nearly 17 million people through some form of aligned contractual arrangement, including full-risk, shared-risk and bundled episode-of-care and performance incentive payment approaches. As of December 31, 2018, our contracts with value-based elements totaled $74 billion in annual spending, including $18 billion through risk-transfer agreements.
Tax Reform. Tax Reform was enacted by the U.S federal government in December 2017, changing existing United States tax law, including reducing the U.S. corporate income tax rate. In 2018, the impact of Tax Reform was partially offset by the return of the nondeductible Health Insurance Industry Tax.
Cash flows from operations were $15.7 billion, an increase of 16%.
The overall increase in people served through risk-based benefit plans in the commercial group market was due to growth in services to small groups. Fee-based commercial group business declined primarily due to customers converting their retirees to Medicare Advantage plans, as well as certain customers expanding the number of carriers and reconfiguring geographies served. Medicare Advantage increased year-over-year due to growth in people served through individual and employer-sponsored group Medicare Advantage plans. The decrease in people served through Medicaid was primarily driven by states adding new carriers to existing programs, reduced enrollment from state efforts to manage eligibility status and the sale of our New Mexico Medicaid plan. Medicare Supplement growth reflected strong customer retention and new sales. International growth was primarily driven by an acquisition in the first quarter.
Investments in financial instruments such as marketable securities and accounts receivable may subject us to concentrations of credit risk. Our investments in marketable securities are managed under an investment policy authorized by our Board of Directors. This policy limits the amounts that may be invested in any one issuer and generally limits our investments to U.S. government and agency securities, state and municipal securities and corporate debt obligations that are investment grade. Concentrations of credit risk with respect to accounts receivable are limited due to the large number of employer groups and other customers that constitute our client base. As of December 31, 2018, there were no significant concentrations of credit risk.
For the years ended December 31, 2018 and 2017, revenue recognized from performance obligations related to prior periods (for example, due to changes in transaction price) was not material.
Other current receivables include amounts due from pharmaceutical manufacturers for rebates and other miscellaneous amounts due to the Company.
For the years ended December 31, 2018 and 2016, no individual factors significantly impacted medical cost reserve development. For the year ended December 31, 2017, medical cost reserve development was primarily driven by lower than expected health system utilization levels.