Investments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt securities
Amortized cost	$ 29,810
Gross unrealized gains	5
Gross unrealized losses	(397)
Fair values	29,418	$ 41,169
Equity securities
Amortized cost		135
Gross unrealized gains		14
Gross unrealized losses		0
Fair values		149
Amortized cost		41,453
Gross unrealized gains		87
Gross unrealized losses		(222)
Fair values		41,318
U.S. Treasury notes [Member]
Debt securities
Amortized cost	2,710	8,313
Gross unrealized gains	0	1
Gross unrealized losses	(84)	(72)
Fair values	2,626	8,242
U.S. Treasury bills [Member]
Debt securities
Amortized cost	5,376
Gross unrealized gains	0
Gross unrealized losses	0
Fair values	5,376
Other government-related debt securities - U.S. [Member]
Debt securities
Amortized cost	112	225
Gross unrealized gains	0	0
Gross unrealized losses	(3)	(2)
Fair values	109	223
Other government-related debt securities - Foreign and other [Member]
Debt securities
Amortized cost	1,048	2,415
Gross unrealized gains	1	18
Gross unrealized losses	(41)	(11)
Fair values	1,008	2,422
Corporate debt securities - Financial [Member]
Debt securities
Amortized cost	2,786	10,089
Gross unrealized gains	0	17
Gross unrealized losses	(89)	(34)
Fair values	2,697	10,072
Corporate debt securities - Industrial [Member]
Debt securities
Amortized cost	2,654	9,688
Gross unrealized gains	4	34
Gross unrealized losses	(80)	(52)
Fair values	2,578	9,670
Corporate debt securities - Other [Member]
Debt securities
Amortized cost	594	1,393
Gross unrealized gains	0	3
Gross unrealized losses	(21)	(6)
Fair values	573	1,390
Residential mortgage-backed securities [Member]
Debt securities
Amortized cost	1,524	2,198
Gross unrealized gains	0	0
Gross unrealized losses	(62)	(30)
Fair values	1,462	2,168
Other mortgage- and asset-backed securities [Member]
Debt securities
Amortized cost	490	2,312
Gross unrealized gains	0	0
Gross unrealized losses	(17)	(15)
Fair values	473	2,297
Money market mutual funds [Member]
Debt securities
Amortized cost	8,955	3,245
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	8,955	3,245
Other short-term interest-bearing securities [Member]
Debt securities
Amortized cost	3,561	1,440
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	$ 3,561	1,440
Total interest-bearing securities [Member]
Debt securities
Amortized cost		41,318
Gross unrealized gains		73
Gross unrealized losses		(222)
Fair values		$ 41,169

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