Derivative Instruments and Other Hedging Activities - Reconciliation of Gross Amounts in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash as collateral for derivative transactions		$ 27,200	$ 11,500
Derivative assets
Derivative instruments		30,086	28,798
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral	[1]	0	0
Net		30,086	28,798
Derivative liabilities
Derivative instruments		24,534	15,411
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral	[1]	(15,212)	(3,735)
Net		9,322	11,676
Written and Purchased Options
Derivative assets
Derivative instruments		6,127	13,387
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral	[1]	0	0
Net		6,127	13,387
Designated as Hedging Instrument | Interest Rate Contracts
Derivative assets
Derivative instruments		0	0
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral	[1]	0	0
Net		0	0
Derivative liabilities
Derivative instruments		575	0
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral		(508)	0	[1]
Net		67	0
Not Designated as Hedging Instruments
Derivative liabilities
Derivative instruments		37,572
Not Designated as Hedging Instruments | Interest Rate Contracts
Derivative assets
Derivative instruments		23,959	15,411
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral	[1]	0	0
Net		23,959	15,411
Derivative liabilities
Derivative instruments		23,959	15,411
Gross amounts not offset in the Balance Sheet, derivatives		0	0
Gross amounts not offset in the Balance Sheet, collateral	[1]	(14,704)	(3,735)
Net		$ 9,255	$ 11,676

[1] Consists of cash collateral recorded at cost, which approximates fair value, and investment securities.