Investments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	$ 32,167	$ 41,453
Gross unrealized gains	11	87
Gross unrealized losses	(508)	(222)
Fair value	31,670	41,318
U.S. Treasury securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	5,207	8,313
Gross unrealized gains	0	1
Gross unrealized losses	(104)	(72)
Fair value	5,103	8,242
Other government-related debt securities - U.S. [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	132	225
Gross unrealized gains	0	0
Gross unrealized losses	(3)	(2)
Fair value	129	223
Other government-related debt securities - Foreign and other [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,714	2,415
Gross unrealized gains	3	18
Gross unrealized losses	(43)	(11)
Fair value	1,674	2,422
Corporate debt securities - Financial [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	6,189	10,089
Gross unrealized gains	1	17
Gross unrealized losses	(137)	(34)
Fair value	6,053	10,072
Corporate debt securities - Industrial [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	5,687	9,688
Gross unrealized gains	6	34
Gross unrealized losses	(128)	(52)
Fair value	5,565	9,670
Corporate debt securities - Other [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	916	1,393
Gross unrealized gains	1	3
Gross unrealized losses	(21)	(6)
Fair value	896	1,390
Residential mortgage-backed securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,745	2,198
Gross unrealized gains	0	0
Gross unrealized losses	(46)	(30)
Fair value	1,699	2,168
Other mortgage- and asset-backed securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	1,293	2,312
Gross unrealized gains	0	0
Gross unrealized losses	(26)	(15)
Fair value	1,267	2,297
Money market mutual funds [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	9,234	3,245
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	9,234	3,245
Other short-term interest-bearing securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	50	1,440
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	50	1,440
Total interest-bearing securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost		41,318
Gross unrealized gains		73
Gross unrealized losses		(222)
Fair value	$ 31,670	41,169
Equity securities [Member]
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost		135
Gross unrealized gains		14
Gross unrealized losses		0
Fair value		$ 149