Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Less: Valuation Allowance	$ 26,000,000	$ 0
Credit carryforward	130,000,000	88,000,000
Deferred tax asset	1,436,000,000	1,077,000,000
Unrecognized tax benefits	53,000,000	12,000,000	$ 6,000,000
Net interest expenses (credits)	(400,000)	(600,000)	(10,600,000)
Recovery of penalties related to unrecognized tax benefits in income tax expense	0	300,000	$ 0
Accrued interest payable	3,000,000	900,000
Reduction in federal tax credit carryforward	10,100,000	2,700,000
Maximum [Member]
Income Taxes [Line Items]
Less: Valuation Allowance	26,000,000
Federal [Member]
Income Taxes [Line Items]
Federal net operating loss carryforwards	84,000,000
Credit carryforward	100,000,000
Capital loss carryforwards	23,000,000
State [Member]
Income Taxes [Line Items]
Credit carryforward	12,000,000
Capital loss carryforwards	3,000,000
State net operating loss carryforwards	79,000,000
Deferred tax asset	15,000,000
Accrued state tax penalties	1,000,000	$ 0
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