Income Taxes - Income Taxes Included in Income Statement (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Currently payable:
Federal	$ 184,000,000	$ 334,000,000	$ 149,000,000
State	62,000,000	0	19,000,000
Total currently payable	246,000,000	334,000,000	168,000,000
Deferred:
Federal	117,000,000	274,000,000	13,000,000
State	(19,000,000)	29,000,000	(2,000,000)
Total deferred	98,000,000	303,000,000	11,000,000
Total income tax expense (benefit)	344,000,000	637,000,000	179,000,000
Income tax (benefit) expense on securities transactions	0	0	0
Equity and gross receipts based taxes assessed in lieu of income tax recorded in noninterest expense	$ 15,000,000	$ 22,000,000	$ 18,000,000