Stock-Based Compensation - Activity and Pricing Information for Nonvested Shares Granted Under Deferred Compensation Plans and Other Restricted Stock Awards (Detail) - Deferred Compensation Plans and Other Restricted Stock Award [Member] - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Number of Nonvested Shares Outstanding, Beginning Balance	3,131,398
Number of Nonvested Shares, Granted	2,141,523
Number of Nonvested Shares, First Niagara acquisition	104,683
Number of Nonvested Shares, Dividend Equivalents	9,310
Number of Nonvested Shares, Vested	(1,401,650)
Number of Nonvested Shares, Forfeited	(127,112)
Number of Nonvested Shares Outstanding, Ending Balance	3,858,152	3,131,398
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted-Average Grant-Date Fair Value Outstanding, Beginning Balance (in dollars per share)	$ 12.47
Weighted-Average Grant Date Fair Value, Granted (in dollars per share)	11.46	$ 14.22	$ 13.53
Weighted-Average Grant-Date Fair Value, First Niagara acquisition (in dollars per share)	11.57
Weighted-Average Grant Date Fair Value, Dividend Equivalents	12.58
Weighted-Average Grant-Date Fair Value, Vested (in dollars per share)	11.66
Weighted-Average Grant-Date Fair Value, Forfeited (in dollars per share)	11.24
Weighted-Average Grant-Date Fair Value Outstanding, Ending Balance (in dollars per share)	$ 12.22	$ 12.47