Income Taxes - Schedule of Reconciliation Between Provision for Income Taxes and Amount Computed by Applying Federal Statutory Income Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rate of 35%	$ 1,164	$ 1,225	$ 1,021
Increase (decrease) in provision for income taxes as a result of:
State income taxes, net of federal tax benefit	77	73	72
Affordable housing projects proportional amortization	236	205	181
Affordable housing projects tax credits and other tax benefits	(319)	(279)	(249)
Tax exempt income	(139)	(151)	(129)
Federal tax reform impact	(43)	0	0
Excess tax benefits for equity-based compensation	(52)	0	0
Adjustments for uncertain tax positions	0	(6)	(107)
Other, net	(13)	(9)	5
Provision for income taxes	$ 911	$ 1,058	$ 794
Effective income tax rate	27.40%	30.20%	27.20%