Available-for-sale investments (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security
Amortized cost	$ 34,244	$ 31,366
Gross unrealized gains	498	75
Gross unrealized losses	(52)	(329)
Estimated fair value	34,690	31,112
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	7,916	4,298
Gross unrealized gains	104	0
Gross unrealized losses	0	(24)
Estimated fair value	8,020	4,274
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	345	536
Gross unrealized gains	2	0
Gross unrealized losses	0	(2)
Estimated fair value	347	534
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	1,814	1,768
Gross unrealized gains	54	7
Gross unrealized losses	(2)	(36)
Estimated fair value	1,866	1,739
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	8,120	7,904
Gross unrealized gains	121	7
Gross unrealized losses	(3)	(40)
Estimated fair value	8,238	7,871
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	8,345	7,961
Gross unrealized gains	141	11
Gross unrealized losses	(13)	(136)
Estimated fair value	8,473	7,836
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	1,002	905
Gross unrealized gains	17	1
Gross unrealized losses	(1)	(21)
Estimated fair value	1,018	885
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	1,729	1,484
Gross unrealized gains	15	1
Gross unrealized losses	(6)	(15)
Estimated fair value	1,738	1,470
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	2,197	2,524
Gross unrealized gains	10	0
Gross unrealized losses	(27)	(55)
Estimated fair value	2,180	2,469
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	1,673	3,370
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	1,673	3,370
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	1,014	528
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	1,014	528
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	34,155	31,278
Gross unrealized gains	464	27
Gross unrealized losses	(52)	(329)
Estimated fair value	34,567	30,976
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	89	88
Gross unrealized gains	34	48
Gross unrealized losses	0	0
Estimated fair value	$ 123	$ 136