Investments (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 7,291,000,000	$ 8,206,000,000
Gross unrealized gains	3,000,000	28,000,000
Gross unrealized losses	0	0
Fair values	7,294,000,000	8,234,000,000
U.S. Treasury notes [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	173,000,000	359,000,000
Gross unrealized gains	3,000,000	1,000,000
Gross unrealized losses	0	0
Fair values	176,000,000	360,000,000
U.S. Treasury bills [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	900,000,000	0
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	900,000,000	0
Corporate debt securities - Financial [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	12,000,000	1,108,000,000
Gross unrealized gains	0	13,000,000
Gross unrealized losses	0	0
Fair values	12,000,000	1,121,000,000
Corporate debt securities - Industrial [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	12,000,000	824,000,000
Gross unrealized gains	0	10,000,000
Gross unrealized losses	0	0
Fair values	12,000,000	834,000,000
Corporate debt securities - Other [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0	195,000,000
Gross unrealized gains	0	3,000,000
Gross unrealized losses	0	0
Fair values	0	198,000,000
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0	181,000,000
Gross unrealized gains	0	1,000,000
Gross unrealized losses	0	0
Fair values	0	182,000,000
Money market mutual funds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	5,762,000,000	5,250,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	5,762,000,000	5,250,000,000
Other short-term interest-bearing securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	432,000,000	289,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	$ 432,000,000	$ 289,000,000