Derivative Financial Instruments - Aggregate Fair Value and Notional Amount of Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Fair Value	$ (27,857)	$ (41,914)
Notional Amount	6,392,016	3,663,084
Derivative Liabilities
Derivative [Line Items]
Fair Value	(54,109)	(58,331)
Notional Amount	2,877,936	1,675,617
Derivative Assets
Derivative [Line Items]
Fair Value	26,252	16,417
Notional Amount	3,514,080	1,987,467
Interest rate swaps | Derivative Liabilities
Derivative [Line Items]
Fair Value	(4,172)	(1,328)
Notional Amount	532,500	206,000
Interest rate swaps | Cash Flow Hedging | Derivative Liabilities
Derivative [Line Items]
Fair Value	(39,810)	(46,845)
Notional Amount		139,500
Interest rate swaps | Derivative Assets
Derivative [Line Items]
Fair Value	294	0
Notional Amount	50,000	0
TBAs | Derivative Liabilities
Derivative [Line Items]
Fair Value	(5,466)	(9,506)
Notional Amount	1,290,500	1,110,000
TBAs | Derivative Assets
Derivative [Line Items]
Fair Value	7,625	6,654
Notional Amount	1,246,400	1,074,000
Futures | Derivative Liabilities
Derivative [Line Items]
Fair Value	(260)	(372)
Notional Amount		90,000
Futures | Derivative Assets
Derivative [Line Items]
Fair Value	0	0
Notional Amount	0	0
Swaptions | Derivative Assets
Derivative [Line Items]
Fair Value	8,726	7,006
Notional Amount	1,185,000	575,000
Credit Default Index Swaps | Derivative Assets
Derivative [Line Items]
Fair Value	3,792	1,597
Notional Amount	100,000	50,000
Loan Purchase Commitments | Derivative Liabilities
Derivative [Line Items]
Fair Value	(4,401)	(41)
Notional Amount	861,436	27,324
Loan Purchase Commitments | Derivative Assets
Derivative [Line Items]
Fair Value	5,006	1,160
Notional Amount	777,361	288,467
Loan forward Sales Commitments | Derivative Liabilities
Derivative [Line Items]
Fair Value	0	(239)
Notional Amount	0	102,793
Loan forward Sales Commitments | Derivative Assets
Derivative [Line Items]
Fair Value	809	0
Notional Amount	$ 155,319	$ 0