Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			$ 23,221
Other comprehensive income (loss), net of taxes	$ 59	$ (305)	470	$ 817
Shareholders' equity, end of period	23,738	24,439	23,738	24,439
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	64	(464)	660	1,248
Income tax expense (benefit)	5	(159)	190	431
Other comprehensive income (loss), net of taxes	59	(305)	470	817
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(755)	(157)
Other comprehensive income before reclassifications			532
Amounts reclassified from accumulated other comprehensive income			(62)
Other comprehensive income (loss), net of taxes	59	(305)	470	817
Shareholders' equity, end of period	(285)	660	(285)	660
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	64	(464)	660	1,248
Income tax expense (benefit)	5	(159)	190	431
Other comprehensive income (loss), net of taxes	59	(305)	470	817
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			377
Amounts reclassified from accumulated other comprehensive income			(103)
Other comprehensive income (loss), net of taxes	(30)	(296)	274	745
Shareholders' equity, end of period	802		802
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(42)	(455)	429	1,138
Income tax expense (benefit)	(12)	(159)	155	393
Other comprehensive income (loss), net of taxes	(30)	(296)	274	745
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
Amounts reclassified from accumulated other comprehensive income			1
Other comprehensive income (loss), net of taxes	1	4	2	15
Shareholders' equity, end of period	204		204
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	2	6	4	23
Income tax expense (benefit)	1	2	2	8
Other comprehensive income (loss), net of taxes	1	4	2	15
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(703)
Other comprehensive income before reclassifications			(23)
Amounts reclassified from accumulated other comprehensive income			40
Other comprehensive income (loss), net of taxes	(6)	10	17	33
Shareholders' equity, end of period	(686)		(686)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(9)	16	25	50
Income tax expense (benefit)	(3)	6	8	17
Other comprehensive income (loss), net of taxes	(6)	10	17	33
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(782)
Other comprehensive income before reclassifications			177
Other comprehensive income (loss), net of taxes	94	(23)	177	24
Shareholders' equity, end of period	(605)		(605)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	113	(31)	202	37
Income tax expense (benefit)	19	(8)	25	13
Other comprehensive income (loss), net of taxes	$ 94	$ (23)	$ 177	$ 24