Investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt securities
Amortized cost	$ 29,324
Gross unrealized gains	6
Gross unrealized losses	(494)
Fair values	28,836	$ 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 securities [Member]
Debt securities
Amortized cost	3,477	8,313
Gross unrealized gains	0	1
Gross unrealized losses	(86)	(72)
Fair values	3,391	8,242
Other government-related debt securities - U.S. [Member]
Debt securities
Amortized cost	132	225
Gross unrealized gains	0	0
Gross unrealized losses	(4)	(2)
Fair values	128	223
Other government-related debt securities - Foreign and other [Member]
Debt securities
Amortized cost	1,451	2,415
Gross unrealized gains	1	18
Gross unrealized losses	(62)	(11)
Fair values	1,390	2,422
Corporate debt securities - Financial [Member]
Debt securities
Amortized cost	4,091	10,089
Gross unrealized gains	1	17
Gross unrealized losses	(124)	(34)
Fair values	3,968	10,072
Corporate debt securities - Industrial [Member]
Debt securities
Amortized cost	3,957	9,688
Gross unrealized gains	4	34
Gross unrealized losses	(122)	(52)
Fair values	3,839	9,670
Corporate debt securities - Other [Member]
Debt securities
Amortized cost	699	1,393
Gross unrealized gains	0	3
Gross unrealized losses	(23)	(6)
Fair values	676	1,390
Residential mortgage-backed securities [Member]
Debt securities
Amortized cost	1,632	2,198
Gross unrealized gains	0	0
Gross unrealized losses	(53)	(30)
Fair values	1,579	2,168
Other mortgage- and asset-backed securities [Member]
Debt securities
Amortized cost	672	2,312
Gross unrealized gains	0	0
Gross unrealized losses	(20)	(15)
Fair values	652	2,297
Money market mutual funds [Member]
Debt securities
Amortized cost	7,341	3,245
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	7,341	3,245
Other short-term interest-bearing securities [Member]
Debt securities
Amortized cost	5,872	1,440
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair values	$ 5,872	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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