Other Comprehensive Income and Accumulated Other Comprehensive Income (details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	$ 24,836	$ 24,796
Other comprehensive income (loss), net of taxes	(1,037)	70	$ (1,426)
Shareholders' equity, end of year	23,598	24,836	24,796
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(1,429)	195	(2,196)
Income tax expense (benefit), other comprehensive income (loss)	(392)	125	(770)
Other comprehensive income (loss), net of taxes	(1,037)	70	(1,426)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	880	810	2,236
Other comprehensive income (loss) before reclassifications	(1,089)	52	(1,463)
Amounts reclassified from accumulated other comprehensive income	52	18	37
Other comprehensive income (loss), net of taxes	(1,037)	70	(1,426)
Shareholders' equity, end of year	(157)	880	810
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(1,429)	195	(2,196)
Income tax expense (benefit), other comprehensive income (loss)	(392)	125	(770)
Other comprehensive income (loss), net of taxes	(1,037)	70	(1,426)
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	1,768	1,125	2,908
Other comprehensive income (loss) before reclassifications	(641)	667	(1,740)
Amounts reclassified from accumulated other comprehensive income	(27)	(24)	(43)
Other comprehensive income (loss), net of taxes	(668)	643	(1,783)
Shareholders' equity, end of year	1,100	1,768	1,125
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(1,020)	976	(2,734)
Income tax expense (benefit), other comprehensive income (loss)	(352)	333	(951)
Other comprehensive income (loss), net of taxes	(668)	643	(1,783)
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	198	197	195
Other comprehensive income (loss) before reclassifications	(11)	(2)	(2)
Amounts reclassified from accumulated other comprehensive income	2	3	4
Other comprehensive income (loss), net of taxes	(9)	1	2
Shareholders' equity, end of year	189	198	197
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(14)	2	3
Income tax expense (benefit), other comprehensive income (loss)	(5)	1	1
Other comprehensive income (loss), net of taxes	(9)	1	2
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	(755)	(431)	(857)
Other comprehensive income (loss) before reclassifications	(18)	(363)	358
Amounts reclassified from accumulated other comprehensive income	60	39	68
Other comprehensive income (loss), net of taxes	42	(324)	426
Shareholders' equity, end of year	(713)	(755)	(431)
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	66	(494)	647
Income tax expense (benefit), other comprehensive income (loss)	24	(170)	221
Other comprehensive income (loss), net of taxes	42	(324)	426
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Shareholders' equity, beginning of year	(331)	(81)	(10)
Other comprehensive income (loss) before reclassifications	(419)	(250)	(79)
Amounts reclassified from accumulated other comprehensive income	17		8
Other comprehensive income (loss), net of taxes	(402)	(250)	(71)
Shareholders' equity, end of year	(733)	(331)	(81)
Pretax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(461)	(289)	(112)
Income tax expense (benefit), other comprehensive income (loss)	(59)	(39)	(41)
Other comprehensive income (loss), net of taxes	$ (402)	$ (250)	$ (71)