Shareholders' Equity - Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 22,317	$ 21,219
Ending balance	22,399	21,294
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,009)	(2,040)
Other comprehensive income (loss) before reclassifications	(50)	253
Amounts reclassified into (out of) earnings	(15)	(18)
Other comprehensive income (loss)	(65)	235
Ending balance	(1,074)	(1,805)
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(56)	229
Other comprehensive income (loss) before reclassifications	(97)	(70)
Amounts reclassified into (out of) earnings	0	0
Other comprehensive income (loss)	(97)	(70)
Ending balance	(153)	159
Net Unrealized Gains (Losses) on Available-for-Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	103	(286)
Other comprehensive income (loss) before reclassifications	(130)	231
Amounts reclassified into (out of) earnings	(1)	(24)
Other comprehensive income (loss)	(131)	207
Ending balance	(28)	(79)
Net Unrealized Losses on Hedges of Net Investments in Non-U.S. Subsidiaries
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(65)	95
Other comprehensive income (loss) before reclassifications	(36)	(14)
Amounts reclassified into (out of) earnings	0	0
Other comprehensive income (loss)	(36)	(14)
Ending balance	(101)	81
Other-Than-Temporary Impairment on Held-to-Maturity Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6)	(9)
Other comprehensive income (loss) before reclassifications	1	1
Amounts reclassified into (out of) earnings	(1)	0
Other comprehensive income (loss)	0	1
Ending balance	(6)	(8)
Net Unrealized Losses on Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(170)	(194)
Other comprehensive income (loss) before reclassifications	25	0
Amounts reclassified into (out of) earnings	(13)	6
Other comprehensive income (loss)	12	6
Ending balance	(158)	(188)
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(815)	(1,875)
Other comprehensive income (loss) before reclassifications	187	105
Amounts reclassified into (out of) earnings	0	0
Other comprehensive income (loss)	187	105
Ending balance	$ (628)	$ (1,770)