Other Comprehensive Income and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	$ 23,731
Other comprehensive income (loss), net of taxes	(933)	$ 140
Balance, end of period	22,979	23,612
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(1,177)	202
Income tax expense (benefit)	(244)	62
Other comprehensive income (loss), net of taxes	(933)	140
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(343)	(755)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, pre-tax			$ (34)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, tax			(12)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, net of income taxes			(22)	$ (22)
Reclassification of certain tax effects from adoption of updated accounting guidance	(24)
Effect of adoption of updated accounting guidance, net of tax, total			(46)
Other comprehensive income (loss) before reclassifications	(950)
Amounts reclassified from accumulated other comprehensive income	17
Other comprehensive income (loss), net of taxes	(933)	140
Balance, end of period	(1,322)	(615)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(1,177)	202
Income tax expense (benefit)	(244)	62
Other comprehensive income (loss), net of taxes	(933)	140
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]
Balance, beginning of year	747
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, pre-tax			(34)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, tax			(12)
Cumulative effect of adoption of updated accounting guidance for equity financial instruments, net of income taxes			(22)
Reclassification of certain tax effects from adoption of updated accounting guidance	145
Effect of adoption of updated accounting guidance, net of tax, total			123
Other comprehensive income (loss) before reclassifications	(950)
Other comprehensive income (loss), net of taxes	(950)	93
Balance, end of period	(80)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(1,203)	144
Income tax expense (benefit)	(253)	51
Other comprehensive income (loss), net of taxes	(950)	93
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]
Balance, beginning of year	207
Reclassification of certain tax effects from adoption of updated accounting guidance	7
Effect of adoption of updated accounting guidance, net of tax, total			7
Other comprehensive income (loss) before reclassifications	(1)
Other comprehensive income (loss), net of taxes	(1)
Balance, end of period	213
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	(2)
Income tax expense (benefit)	(1)
Other comprehensive income (loss), net of taxes	(1)
Net Benefit Plan Assets and Obligations Recognized in Shareholders' Equity [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(686)
Reclassification of certain tax effects from adoption of updated accounting guidance	(141)
Effect of adoption of updated accounting guidance, net of tax, total			(141)
Amounts reclassified from accumulated other comprehensive income	17
Other comprehensive income (loss), net of taxes	17	12
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	22	17
Income tax expense (benefit)	5	5
Other comprehensive income (loss), net of taxes	17	12
Net Unrealized Foreign Currency Translation [Member]
Accumulated other comprehensive income [Roll Forward]
Balance, beginning of year	(611)
Reclassification of certain tax effects from adoption of updated accounting guidance	(35)
Effect of adoption of updated accounting guidance, net of tax, total			$ (35)
Other comprehensive income (loss) before reclassifications	1
Other comprehensive income (loss), net of taxes	1	35
Balance, end of period	(645)
Pre-tax components of other comprehensive income (loss) and related tax expense (benefit)
Other comprehensive income (loss) before income taxes	6	41
Income tax expense (benefit)	5	6
Other comprehensive income (loss), net of taxes	$ 1	$ 35