Significant Accounting Policies - Receivables (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	$ 1,603	$ 1,406
Government receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	1,167	1,056
Pharmacy rebate receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	160	150
Health insurer fee reimbursement receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	71	5
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables	$ 205	$ 195