Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year			$ 22,894
Other comprehensive income (loss), net of taxes	$ 888	$ (366)	2,065	$ (1,299)
Balance, end of period	25,321	22,623	25,321	22,623
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	1,123	(447)	2,606	(1,624)
Income tax expense (benefit)	235	(81)	541	(325)
Other comprehensive income (loss), net of taxes	888	(366)	2,065	(1,299)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(682)	(1,322)	(1,859)	(343)
Other comprehensive income (loss) before reclassifications	887		2,067
Amounts reclassified from accumulated other comprehensive income	1		(2)
Other comprehensive income (loss), net of taxes	888	(366)	2,065	(1,299)
Balance, end of period	206	(1,688)	206	(1,688)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	1,123	(447)	2,606	(1,624)
Income tax expense (benefit)	235	(81)	541	(325)
Other comprehensive income (loss), net of taxes	888	(366)	2,065	(1,299)
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	810		(306)
Other comprehensive income (loss) before reclassifications	884		2,013
Amounts reclassified from accumulated other comprehensive income	(10)		(23)
Other comprehensive income (loss), net of taxes	874	(235)	1,990	(1,185)
Balance, end of period	1,684		1,684
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	1,108	(298)	2,524	(1,501)
Income tax expense (benefit)	234	(63)	534	(316)
Other comprehensive income (loss), net of taxes	874	(235)	1,990	(1,185)
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	197		193
Other comprehensive income (loss) before reclassifications	(3)		1
Amounts reclassified from accumulated other comprehensive income	0		0
Other comprehensive income (loss), net of taxes	(3)	(10)	1	(11)
Balance, end of period	194		194
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(4)	(12)	1	(14)
Income tax expense (benefit)	(1)	(2)	0	(3)
Other comprehensive income (loss), net of taxes	(3)	(10)	1	(11)
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(863)		(873)
Other comprehensive income (loss) before reclassifications	0		0
Amounts reclassified from accumulated other comprehensive income	11		21
Other comprehensive income (loss), net of taxes	11	17	21	34
Balance, end of period	(852)		(852)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	14	21	26	43
Income tax expense (benefit)	3	4	5	9
Other comprehensive income (loss), net of taxes	11	17	21	34
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(826)		(873)
Other comprehensive income (loss) before reclassifications	6		53
Amounts reclassified from accumulated other comprehensive income	0		0
Other comprehensive income (loss), net of taxes	6	(138)	53	(137)
Balance, end of period	(820)		(820)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	5	(158)	55	(152)
Income tax expense (benefit)	(1)	(20)	2	(15)
Other comprehensive income (loss), net of taxes	$ 6	$ (138)	$ 53	$ (137)