Shareholders' Equity - Additional Information (Detail)			3 Months Ended		9 Months Ended
	Sep. 09, 2016 USD ($)	Aug. 01, 2016 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares	Jun. 29, 2016 USD ($)	Dec. 31, 2015 USD ($) shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cash dividends declared on common shares (in usd per share)			$ 0.085	$ 0.075	$ 0.245	$ 0.215
Preferred stock | $			$ 1,165,000,000	$ 290,000,000	$ 1,165,000,000	$ 290,000,000		$ 290,000,000
Preferred stock, shares outstanding (in shares) | shares			2,900,234		2,900,234
Dividend payable | $					$ 224,000,000	$ 199,000,000
First Niagara Bank, N.A. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Acquiree shares equivalent of acquirer common stock | shares		0.680
Cash payable per acquiree common stock		$ 2.30
Cash payments to First Niagara stockholders | $		$ 811,000,000
Series A Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock | $			$ 290,000,000,000		$ 290,000,000,000
Preferred stock, dividend rate					7.75%
Preferred stock, par value (in usd per share)			$ 1		$ 1
Preferred stock, liquidation preference			$ 100		$ 100
Preferred stock, shares authorized (in shares) | shares			7,475,000	7,475,000	7,475,000	7,475,000		7,475,000
Preferred stock, shares outstanding (in shares) | shares				2,900,234		2,900,234		2,900,234
Dividend payable per share			$ 1.9375		$ 1.9375
Dividend payable | $					$ 5,600,000
Series C Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock | $		$ 350,000,000
Preferred stock, par value (in usd per share)			1		$ 1
Preferred stock, liquidation preference			$ 25		$ 25
Preferred stock, shares authorized (in shares) | shares			14,000,000		14,000,000
Preferred stock, shares outstanding (in shares) | shares			14,000,000		14,000,000
Depository Shares [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Shares issued | $	$ 525,000,000
Series D Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, par value (in usd per share)			$ 1		$ 1
Preferred stock, liquidation preference			$ 25,000		$ 25,000
Preferred stock, shares authorized (in shares) | shares			21,000		21,000
Preferred stock, shares outstanding (in shares) | shares			21,000		21,000
Depository receipt ratio	0.04
2016 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Authorized amount of share repurchases | $							$ 350,000,000
Common shares repurchased, value | $			$ 65,000,000
2016 [Member] | 2016 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cash dividends declared on common shares (in usd per share)			$ 0.085
2017 [Member] | 2016 Capital Plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Proposed increase in cash dividends declared on common shares, per share					$ 0.095
Common Stock [Member] | First Niagara Bank, N.A. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Number of shares received by acquiree | shares		240,000,000