Available-for-sale investments (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	$ 29,668	$ 26,763
Gross unrealized gains	176	143
Gross unrealized losses	(155)	(174)
Estimated fair value	29,689	26,732
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	3,604	3,632
Gross unrealized gains	14	22
Gross unrealized losses	(8)	(8)
Estimated fair value	3,610	3,646
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	551	530
Gross unrealized gains	1	1
Gross unrealized losses	(1)	(3)
Estimated fair value	551	528
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	1,718	1,572
Gross unrealized gains	19	21
Gross unrealized losses	(14)	(24)
Estimated fair value	1,723	1,569
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	7,425	6,036
Gross unrealized gains	22	21
Gross unrealized losses	(21)	(16)
Estimated fair value	7,426	6,041
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	7,657	6,394
Gross unrealized gains	27	23
Gross unrealized losses	(55)	(66)
Estimated fair value	7,629	6,351
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	830	650
Gross unrealized gains	3	3
Gross unrealized losses	(4)	(4)
Estimated fair value	829	649
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	1,498	1,708
Gross unrealized gains	7	4
Gross unrealized losses	(8)	(10)
Estimated fair value	1,497	1,702
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	2,053	1,837
Gross unrealized gains	1	0
Gross unrealized losses	(40)	(41)
Estimated fair value	2,014	1,796
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	2,951	3,004
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	2,951	3,004
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	1,284	1,302
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	1,284	1,302
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	29,571	26,665
Gross unrealized gains	94	95
Gross unrealized losses	(151)	(172)
Estimated fair value	29,514	26,588
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	97	98
Gross unrealized gains	82	48
Gross unrealized losses	(4)	(2)
Estimated fair value	$ 175	$ 144