Investments - Schedule (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Debt securities
Amortized cost	$ 8,206,000,000	$ 29,064,000,000
Gross unrealized gains	28,000,000	1,000,000
Gross unrealized losses	0	(341,000,000)
Fair values	8,234,000,000	28,724,000,000
U.S. Treasury notes [Member]
Debt securities
Amortized cost	359,000,000	2,710,000,000
Gross unrealized gains	1,000,000	0
Gross unrealized losses	0	(47,000,000)
Fair values	360,000,000	2,663,000,000
U.S. Treasury bills [Member]
Debt securities
Amortized cost	0	8,191,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	0	8,191,000,000
Other government-related debt securities - U.S. [Member]
Debt securities
Amortized cost	0	112,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	(2,000,000)
Fair values	0	110,000,000
Other government-related debt securities - Foreign and other [Member]
Debt securities
Amortized cost	0	972,000,000
Gross unrealized gains	0	1,000,000
Gross unrealized losses	0	(41,000,000)
Fair values	0	932,000,000
Corporate debt securities - Financial [Member]
Debt securities
Amortized cost	1,108,000,000	2,778,000,000
Gross unrealized gains	13,000,000	0
Gross unrealized losses	0	(81,000,000)
Fair values	1,121,000,000	2,697,000,000
Corporate debt securities - Industrial [Member]
Debt securities
Amortized cost	824,000,000	2,603,000,000
Gross unrealized gains	10,000,000	0
Gross unrealized losses	0	(99,000,000)
Fair values	834,000,000	2,504,000,000
Corporate debt securities - Other [Member]
Debt securities
Amortized cost	195,000,000	583,000,000
Gross unrealized gains	3,000,000	0
Gross unrealized losses	0	(21,000,000)
Fair values	198,000,000	562,000,000
Residential mortgage-backed securities [Member]
Debt securities
Amortized cost	181,000,000	1,458,000,000
Gross unrealized gains	1,000,000	0
Gross unrealized losses	0	(36,000,000)
Fair values	182,000,000	1,422,000,000
Other mortgage- and asset-backed securities [Member]
Debt securities
Amortized cost	0	483,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	(14,000,000)
Fair values	0	469,000,000
Money market mutual funds [Member]
Debt securities
Amortized cost	5,250,000,000	5,659,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	5,250,000,000	5,659,000,000
Other short-term interest-bearing securities [Member]
Debt securities
Amortized cost	289,000,000	3,515,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	$ 289,000,000	$ 3,515,000,000