Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	$ 23,221	$ 23,598
Other comprehensive income (loss), net of taxes	412	(598)	$ (1,037)
Shareholders' equity, end of year	23,731	23,221	23,598
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	522	(887)	(1,429)
Income tax expense (benefit)	110	(289)	(392)
Other comprehensive income (loss), net of taxes	412	(598)	(1,037)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	(755)	(157)	880
Other comprehensive income (loss) before reclassifications	518	(605)	(1,089)
Amounts reclassified from accumulated other comprehensive income	(106)	7	52
Other comprehensive income (loss), net of taxes	412	(598)	(1,037)
Shareholders' equity, end of year	(343)	(755)	(157)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	522	(887)	(1,429)
Income tax expense (benefit)	110	(289)	(392)
Other comprehensive income (loss), net of taxes	412	(598)	(1,037)
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	1,100	1,768
Other comprehensive income (loss) before reclassifications	367	(530)	(641)
Amounts reclassified from accumulated other comprehensive income	(148)	(42)	(27)
Other comprehensive income (loss), net of taxes	219	(572)	(668)
Shareholders' equity, end of year	747	528	1,100
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	294	(883)	(1,020)
Income tax expense (benefit)	75	(311)	(352)
Other comprehensive income (loss), net of taxes	219	(572)	(668)
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	189	198
Other comprehensive income (loss) before reclassifications	4	4	(11)
Amounts reclassified from accumulated other comprehensive income	1	9	2
Other comprehensive income (loss), net of taxes	5	13	(9)
Shareholders' equity, end of year	207	202	189
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	8	21	(14)
Income tax expense (benefit)	3	8	(5)
Other comprehensive income (loss), net of taxes	5	13	(9)
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	(703)	(713)	(755)
Other comprehensive income (loss) before reclassifications	(24)	(30)	(18)
Amounts reclassified from accumulated other comprehensive income	41	40	60
Other comprehensive income (loss), net of taxes	17	10	42
Shareholders' equity, end of year	(686)	(703)	(713)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	29	16	66
Income tax expense (benefit)	12	6	24
Other comprehensive income (loss), net of taxes	17	10	42
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	(782)	(733)	(331)
Other comprehensive income (loss) before reclassifications	171	(49)	(419)
Amounts reclassified from accumulated other comprehensive income			17
Other comprehensive income (loss), net of taxes	171	(49)	(402)
Shareholders' equity, end of year	(611)	(782)	(733)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	191	(41)	(461)
Income tax expense (benefit)	20	8	(59)
Other comprehensive income (loss), net of taxes	$ 171	$ (49)	$ (402)