Derivative Instruments and Other Hedging Activities - Effect of Derivatives on Consolidated Financial Statements (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss)
Gain (loss) on derivatives, net	$ (5,920)	$ (2,469)	$ 2,884		$ (3,140)
Not Designated as Hedging Instruments | Interest Rate Contracts | Other Income
Derivative Instruments, Gain (Loss)
Gain (loss) on derivatives, net	1,130	406	3,069		1,971
Not Designated as Hedging Instruments | Forward Sales Contracts | Mortgage Income
Derivative Instruments, Gain (Loss)
Gain (loss) on derivatives, net	(6,524)	(3,648)	1,526		(7,512)
Not Designated as Hedging Instruments | Written and Purchased Options | Mortgage Income
Derivative Instruments, Gain (Loss)
Gain (loss) on derivatives, net	(526)	773	(1,711)		$ 2,401
Cash Flow Hedging | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Amount of gain (loss) recognized in OCI, net of taxes	(3,412)	0	(374)	$ 0
Amount of gain (loss) reclassified from accumulated OCI into income	0	0	0	0
Amount of gain (loss) recognized in income on derivative	0	0	0	(1)
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Contracts
Derivative Instruments, Gain (Loss)
Amount of gain (loss) recognized in OCI, net of taxes	(3,412)	0	(374)	0
Cash Flow Hedging | Designated as Hedging Instrument | Interest Rate Contracts | Other Income (Expense)
Derivative Instruments, Gain (Loss)
Amount of gain (loss) reclassified from accumulated OCI into income	0	0	0	0
Amount of gain (loss) recognized in income on derivative	$ 0	$ 0	$ 0	$ (1)