Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	$ 22,894
Other comprehensive income (loss), net of taxes	1,177	$ (933)
Balance, end of period	24,340	22,979
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	1,483	(1,177)
Income tax expense (benefit)	306	(244)
Other comprehensive income (loss), net of taxes	1,177	(933)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(1,859)	(343)
Other comprehensive income (loss) before reclassifications	1,180
Amounts reclassified from accumulated other comprehensive income	(3)
Other comprehensive income (loss), net of taxes	1,177	(933)
Balance, end of period	(682)	(1,322)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	1,483	(1,177)
Income tax expense (benefit)	306	(244)
Other comprehensive income (loss), net of taxes	1,177	(933)
Changes in Net Unrealized Gains (Losses) on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(306)
Other comprehensive income (loss) before reclassifications	1,129
Amounts reclassified from accumulated other comprehensive income	(13)
Other comprehensive income (loss), net of taxes	1,116	(950)
Balance, end of period	810
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	1,416	(1,203)
Income tax expense (benefit)	300	(253)
Other comprehensive income (loss), net of taxes	1,116	(950)
Changes in Net Unrealized Gains (Losses) on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	193
Other comprehensive income (loss) before reclassifications	4
Other comprehensive income (loss), net of taxes	4	(1)
Balance, end of period	197
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	5	(2)
Income tax expense (benefit)	1	(1)
Other comprehensive income (loss), net of taxes	4	(1)
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(873)
Amounts reclassified from accumulated other comprehensive income	10
Other comprehensive income (loss), net of taxes	10	17
Balance, end of period	(863)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	12	22
Income tax expense (benefit)	2	5
Other comprehensive income (loss), net of taxes	10	17
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(873)
Other comprehensive income (loss) before reclassifications	47
Other comprehensive income (loss), net of taxes	47	1
Balance, end of period	(826)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	50	6
Income tax expense (benefit)	3	5
Other comprehensive income (loss), net of taxes	$ 47	$ 1