Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Tax reform, change in tax rate, provisional income tax expense			$ 147,000,000
Tax reform, change in enacted tax rate, percent			7.60%
Tax reform, measurement period adjustment		$ 7,000,000
Valuation allowances	$ 0	11,000,000
Credit carryforward	81,000,000	34,000,000
Deferred tax asset	987,000,000	914,000,000
Unrecognized tax benefits	19,000,000	35,000,000	$ 39,000,000
Net interest expenses (credits)	(900,000)	(700,000)	(1,300,000)
Recovery of penalties related to unrecognized tax benefits in income tax expense	0	0	$ 1,000,000
Accrued interest payable	2,000,000	3,000,000
Accrued state tax penalties	0	0
Reduction in federal tax credit carryforward	15,900,000	$ 14,300,000
Maximum [Member]
Income Taxes [Line Items]
Valuation allowances	0
Federal [Member]
Income Taxes [Line Items]
Federal net operating loss carryforwards	43,000,000
Credit carryforward	72,000,000
State [Member]
Income Taxes [Line Items]
State net operating loss carryforwards	40,000,000
Deferred tax asset	1,000,000
First Niagara Bank, N.A. [Member]
Income Taxes [Line Items]
Allocated bad debt deductions for which no income taxes have been recorded	$ 92,000,000