Derivative Instruments and Other Hedging Activities - Schedule of Outstanding Derivative Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2014	Dec. 31, 2015
Fair Value of Derivatives
Derivative assets	$ 48,594		$ 30,486
Derivative liabilities	48,772		24,939
Notional Amount of Derivatives
Derivative assets	1,207,431		1,255,277
Derivative liabilities	1,455,880		950,105
Cash as collateral for derivative transactions	12,200	$ 21,800
Designated as Hedging Instrument
Fair Value of Derivatives
Derivative assets	0		58
Derivative liabilities	6,292		0
Notional Amount of Derivatives
Derivative assets	0		108,500
Derivative liabilities	108,500		0
Designated as Hedging Instrument | Interest Rate Contracts
Notional Amount of Derivatives
Derivative assets	0		108,500
Derivative liabilities	108,500		0
Designated as Hedging Instrument | Interest Rate Contracts | Other Assets
Fair Value of Derivatives
Derivative assets	0		58
Designated as Hedging Instrument | Interest Rate Contracts | Other Liabilities
Fair Value of Derivatives
Derivative liabilities	6,292		0
Not Designated as Hedging Instruments
Fair Value of Derivatives
Derivative assets	48,594		30,428
Derivative liabilities	42,480		24,939
Notional Amount of Derivatives
Derivative assets	1,207,431		1,146,777
Derivative liabilities	1,347,380		950,105
Not Designated as Hedging Instruments | Interest Rate Contracts
Notional Amount of Derivatives
Derivative assets	725,878		590,334
Derivative liabilities	725,878		590,334
Not Designated as Hedging Instruments | Interest Rate Contracts | Other Assets
Fair Value of Derivatives
Derivative assets	33,248		18,077
Not Designated as Hedging Instruments | Interest Rate Contracts | Other Liabilities
Fair Value of Derivatives
Derivative liabilities	33,248		18,077
Not Designated as Hedging Instruments | Foreign Exchange Contracts
Notional Amount of Derivatives
Derivative assets	3,347		4,392
Derivative liabilities	3,347		4,392
Not Designated as Hedging Instruments | Foreign Exchange Contracts | Other Assets
Fair Value of Derivatives
Derivative assets	102		156
Not Designated as Hedging Instruments | Foreign Exchange Contracts | Other Liabilities
Fair Value of Derivatives
Derivative liabilities	86		134
Not Designated as Hedging Instruments | Forward Sales Contracts
Notional Amount of Derivatives
Derivative assets	66,401		223,841
Derivative liabilities	426,251		173,430
Not Designated as Hedging Instruments | Forward Sales Contracts | Other Assets
Fair Value of Derivatives
Derivative assets	44		1,588
Not Designated as Hedging Instruments | Forward Sales Contracts | Other Liabilities
Fair Value of Derivatives
Derivative liabilities	2,282		474
Not Designated as Hedging Instruments | Written and Purchased Options
Notional Amount of Derivatives
Derivative assets	411,805		328,210
Derivative liabilities	191,904		181,949
Not Designated as Hedging Instruments | Written and Purchased Options | Other Assets
Fair Value of Derivatives
Derivative assets	15,200		10,607
Not Designated as Hedging Instruments | Written and Purchased Options | Other Liabilities
Fair Value of Derivatives
Derivative liabilities	$ 6,864		$ 6,254