Summary of Significant Accounting Policies - Offsetting of Financial Assets, Liabilities, and Collateral (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	$ 22,275	$ 15,257
Gross Amounts Offset in Consolidated Balance Sheet		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	22,275	15,257
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(7,386)	(6,975)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(10,884)	(4,735)
Net Amount	4,005	3,547
Gross Amounts of Recognized Liabilities	(1,568,919)	(1,851,876)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(1,568,919)	(1,851,876)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	1,509,549	1,800,800
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	58,395	50,100
Net Amount	(975)	(976)
Interest Rate Swaps
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	17,554	7,006
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	17,554	7,006
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(2,719)	(1,160)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(10,830)	(4,360)
Net Amount	4,005	1,486
Gross Amounts of Recognized Liabilities	(57,581)	(48,173)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(57,581)	(48,173)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	2,719	1,160
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	54,862	47,013
Net Amount	0	0
Credit Default Index Swaps
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets		1,598
Gross Amounts Offset in Consolidated Balance Sheet		0
Net Amounts of Assets Presented in Consolidated Balance Sheet		1,598
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments		0
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received		(375)
Net Amount		1,223
TBAs
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	4,721	6,653
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	4,721	6,653
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(4,667)	(5,815)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(54)	0
Net Amount	0	838
Gross Amounts of Recognized Liabilities	(8,842)	(9,506)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(8,842)	(9,506)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	4,666	5,815
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	3,201	2,715
Net Amount	(975)	(976)
Futures
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(332)	(372)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(332)	(372)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	0	0
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	332	372
Net Amount	0	0
Loan warehouse debt
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(895,895)	(1,185,316)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(895,895)	(1,185,316)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	895,895	1,185,316
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	0	0
Net Amount		0
Security repurchase agreements
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(606,269)	(608,509)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(606,269)	(608,509)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	606,269	608,509
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	0	0
Net Amount	$ 0	$ 0

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Amount, after the effects of master netting arrangements, of derivative assets, securities purchased under agreements to resell and securities borrowed. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount, after effects of master netting arrangements, of derivative assets, securities purchased under agreements to resell and securities borrowed offset against an obligation to return collateral under a master netting arrangement. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets, securities purchased under agreements to resell and securities borrowed.

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Amount, before effects of master netting arrangements, of derivative assets, securities purchased under agreements to resell and securities borrowed. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Amount of liability associated with derivative assets, securities purchased under agreements to resell and securities borrowed.

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