Income Taxes - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Tax reform, provisional income tax expense				$ 161,000,000
Tax reform, provisional income tax expense, low-income housing investments				14,000,000
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
Tax reform, total impact			$ 168,000,000
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate		$ 141,000,000		$ (141,000,000)
Less: Valuation Allowance		15,000,000	11,000,000	15,000,000
Credit carryforward		205,000,000	34,000,000	205,000,000
Deferred tax asset		1,090,000,000	914,000,000	1,090,000,000
Unrecognized tax benefits		39,000,000	35,000,000	39,000,000	$ 53,000,000
Net interest expenses (credits)			(700,000)	(1,300,000)	(400,000)
Recovery of penalties related to unrecognized tax benefits in income tax expense		1,000,000	0	1,000,000	$ 0
Accrued interest payable		4,000,000	3,000,000	4,000,000
Accrued state tax penalties		$ 0	0	0
Reduction in federal tax credit carryforward			14,300,000	$ 17,200,000
Maximum [Member]
Income Taxes [Line Items]
Less: Valuation Allowance			11,000,000
Federal [Member]
Income Taxes [Line Items]
Federal net operating loss carryforwards			61,000,000
Credit carryforward			21,000,000
State [Member]
Income Taxes [Line Items]
Credit carryforward			1,000,000
State net operating loss carryforwards			54,000,000
Deferred tax asset			3,000,000
Capital loss carryforwards			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
Capital Loss Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforward			$ 11,000,000