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, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income taxes at statutory rate	$ 844	$ 853	$ 1,164
Increase (decrease) in provision for income taxes as a result of:
State income taxes, net of federal tax benefit	107	117	77
Income tax credits, net of amortization	86	57	83
Tax-exempt interest	(69)	(90)	(139)
Federal tax reform impact	0	(27)	(43)
Other, net	(14)	7	(65)
Provision for income taxes	$ 782	$ 803	$ 911
Effective Income Tax Rate Reconciliation, Tax Credit, Percent [Abstract]
Statutory federal tax rate	21.00%	21.00%	35.00%
State income taxes, net of federal tax benefit	2.70%	2.90%	2.30%
Income tax credits, net of amortization	2.10%	1.40%	2.50%
Tax-exempt interest	(1.80%)	(2.20%)	(4.20%)
Federal tax reform impact	0	(0.007)	(0.013)
Other, net	(0.30%)	0.20%	(1.90%)
Effective Income Tax Rate Reconciliation, Percent	19.50%	19.80%	27.40%