Subsequent Event - Additional Information (Detail) $ / shares in Units, $ in Millions	Oct. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) Branches	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Subsequent Event [Line Items]
Total assets		$ 95,422	$ 93,821	$ 89,784
Total deposits		$ 71,073	$ 71,998	$ 68,456
First Niagara Financial Group, Inc. [Member]
Subsequent Event [Line Items]
Number of branches | Branches		394
Total assets		$ 39,000
Total deposits		$ 29,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Closing price of common shares | $ / shares	$ 13.38
Subsequent Event [Member] | First Niagara Financial Group, Inc. [Member]
Subsequent Event [Line Items]
Acquiree shares equivalent of acquirer common stock | shares	0.680
Cash payable per acquiree common stock | $ / shares	$ 2.30
Acquiree common stock shares outstanding fully diluted | shares	356,272,000
Total consideration to be paid	$ 4,100