Income Taxes - Narrative (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Tax Cuts and Jobs Act of 2017, provisional income tax expense		$ 161,000,000
Tax Cuts and Jobs Act of 2017, provisional income tax expense, low-income housing investments		14,000,000
Tax Cuts and Jobs Act of 2017, change in tax rate, provisional income tax expense		$ 147,000,000
Effective income tax rate reconciliation, change in enacted tax rate		7.60%
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	15,000,000	$ 26,000,000
Credit carryforward	205,000,000	205,000,000	130,000,000
Deferred tax asset	1,090,000,000	1,090,000,000	1,436,000,000
Unrecognized tax benefits	39,000,000	39,000,000	53,000,000	$ 12,000,000
Net interest expenses (credits)		(1,300,000)	(400,000)	(600,000)
Recovery of penalties related to unrecognized tax benefits in income tax expense	1,000,000	1,000,000	0	$ 300,000
Accrued interest payable	4,000,000	4,000,000	3,000,000
Reduction in federal tax credit carryforward		17,200,000	10,100,000
Maximum [Member]
Income Taxes [Line Items]
Less: Valuation Allowance	15,000,000	15,000,000
Federal [Member]
Income Taxes [Line Items]
Federal net operating loss carryforwards	79,000,000	79,000,000
Credit carryforward	189,000,000	189,000,000
State [Member]
Income Taxes [Line Items]
Credit carryforward	5,000,000	5,000,000
Capital loss carryforwards	2,000,000	2,000,000
State net operating loss carryforwards	72,000,000	72,000,000
Deferred tax asset	7,000,000	7,000,000
Accrued state tax penalties	0	0	$ 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	92,000,000
Capital Loss Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforward	$ 15,000,000	$ 15,000,000