Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			$ 23,221
Other comprehensive income, net of taxes	$ 271	$ 551	411	$ 1,122
Shareholders' equity, end of period	23,858	24,714	23,858	24,714
Pre-tax components of other comprehensive income and related tax expense
Other comprehensive income before income taxes	394	874	596	1,712
Income tax expense	123	323	185	590
Other comprehensive income, net of taxes	271	551	411	1,122
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(755)	(157)
Other comprehensive income before reclassifications			449
Amounts reclassified from accumulated other comprehensive income			(38)
Other comprehensive income, net of taxes	271	551	411	1,122
Shareholders' equity, end of period	(344)	965	(344)	965
Pre-tax components of other comprehensive income and related tax expense
Other comprehensive income before income taxes	394	874	596	1,712
Income tax expense	123	323	185	590
Other comprehensive income, net of taxes	271	551	411	1,122
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			528
Other comprehensive income before reclassifications			365
Amounts reclassified from accumulated other comprehensive income			(61)
Other comprehensive income, net of taxes	211	574	304	1,041
Shareholders' equity, end of period	832		832
Pre-tax components of other comprehensive income and related tax expense
Other comprehensive income before income taxes	327	879	471	1,593
Income tax expense	116	305	167	552
Other comprehensive income, net of taxes	211	574	304	1,041
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			202
Other comprehensive income before reclassifications			1
Other comprehensive income, net of taxes	1	8	1	11
Shareholders' equity, end of period	203		203
Pre-tax components of other comprehensive income and related tax expense
Other comprehensive income before income taxes	2	12	2	17
Income tax expense	1	4	1	6
Other comprehensive income, net of taxes	1	8	1	11
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(703)
Amounts reclassified from accumulated other comprehensive income			23
Other comprehensive income, net of taxes	11	12	23	23
Shareholders' equity, end of period	(680)		(680)
Pre-tax components of other comprehensive income and related tax expense
Other comprehensive income before income taxes	17	18	34	34
Income tax expense	6	6	11	11
Other comprehensive income, net of taxes	11	12	23	23
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(782)
Other comprehensive income before reclassifications			83
Other comprehensive income, net of taxes	48	(43)	83	47
Shareholders' equity, end of period	(699)		(699)
Pre-tax components of other comprehensive income and related tax expense
Other comprehensive income before income taxes	48	(35)	89	68
Income tax expense		8	6	21
Other comprehensive income, net of taxes	$ 48	$ (43)	$ 83	$ 47