Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			$ 23,598
Other comprehensive income (loss), net of taxes	$ (305)	$ (137)	817	$ (848)
Shareholders' equity, end of period	24,439	24,033	24,439	24,033
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(464)	(139)	1,248	(1,178)
Income tax expense (benefit), other comprehensive income (loss)	(159)	(2)	431	(330)
Other comprehensive income (loss), net of taxes	(305)	(137)	817	(848)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(157)	880
Other comprehensive income before reclassifications			814
Amounts reclassified from accumulated other comprehensive income			3
Other comprehensive income (loss), net of taxes	(305)	(137)	817	(848)
Shareholders' equity, end of period	660	32	660	32
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(464)	(139)	1,248	(1,178)
Income tax expense (benefit), other comprehensive income (loss)	(159)	(2)	431	(330)
Other comprehensive income (loss), net of taxes	(305)	(137)	817	(848)
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			1,100
Other comprehensive income before reclassifications			780
Amounts reclassified from accumulated other comprehensive income			(35)
Other comprehensive income (loss), net of taxes	(296)	41	745	(543)
Shareholders' equity, end of period	1,845		1,845
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(455)	67	1,138	(829)
Income tax expense (benefit), other comprehensive income (loss)	(159)	26	393	(286)
Other comprehensive income (loss), net of taxes	(296)	41	745	(543)
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			189
Other comprehensive income before reclassifications			7
Amounts reclassified from accumulated other comprehensive income			8
Other comprehensive income (loss), net of taxes	4	(3)	15	(9)
Shareholders' equity, end of period	204		204
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	6	(3)	23	(13)
Income tax expense (benefit), other comprehensive income (loss)	2		8	(4)
Other comprehensive income (loss), net of taxes	4	(3)	15	(9)
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(713)
Other comprehensive income before reclassifications			3
Amounts reclassified from accumulated other comprehensive income			30
Other comprehensive income (loss), net of taxes	10	15	33	46
Shareholders' equity, end of period	(680)		(680)
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	16	24	50	71
Income tax expense (benefit), other comprehensive income (loss)	6	9	17	25
Other comprehensive income (loss), net of taxes	10	15	33	46
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year			(733)
Other comprehensive income before reclassifications			24
Other comprehensive income (loss), net of taxes	(23)	(190)	24	(342)
Shareholders' equity, end of period	(709)		(709)
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(31)	(227)	37	(407)
Income tax expense (benefit), other comprehensive income (loss)	(8)	(37)	13	(65)
Other comprehensive income (loss), net of taxes	$ (23)	$ (190)	$ 24	$ (342)