Income Taxes - Income Taxes Included in Income Statement (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Currently payable:
Federal	$ 241,000,000	$ 184,000,000	$ 334,000,000
State	20,000,000	62,000,000	0
Total currently payable	261,000,000	246,000,000	334,000,000
Deferred:
Federal	34,000,000	117,000,000	274,000,000
State	19,000,000	(19,000,000)	29,000,000
Total deferred	53,000,000	98,000,000	303,000,000
Total income tax expense (benefit)	314,000,000	344,000,000	637,000,000
Income tax (benefit) expense on securities transactions	5,000,000	0
Equity and gross receipts based taxes assessed in lieu of income tax recorded in noninterest expense	$ 23,000,000	$ 15,000,000	$ 22,000,000