Summary of Significant Accounting Policies - Offsetting of Financial Assets, Liabilities, and Collateral (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	$ 27,031	$ 11,722
Gross Amounts Offset in Consolidated Balance Sheet		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	27,031	11,722
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(20,265)	(7,549)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(4,581)	(2,930)
Net Amount	2,185	1,243
Gross Amounts of Recognized Liabilities	(901,338)	(1,778,711)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(901,338)	(1,778,711)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	824,440	1,724,930
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	76,178	53,167
Net Amount	(720)	(614)
Interest rate agreements
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	24,188	7,781
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	24,188	7,781
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(18,926)	(5,651)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(4,581)	(1,917)
Net Amount	681	213
Gross Amounts of Recognized Liabilities	(92,100)	(58,366)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(92,100)	(58,366)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	18,926	5,651
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	73,174	52,715
Net Amount	0	0
Credit Default Index Swaps
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets		1,207
Gross Amounts Offset in Consolidated Balance Sheet		0
Net Amounts of Assets Presented in Consolidated Balance Sheet		1,207
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments		0
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received		(720)
Net Amount		487
TBAs
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	2,843	2,734
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	2,843	2,734
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(1,339)	(1,898)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	0	(293)
Net Amount	1,504	543
Gross Amounts of Recognized Liabilities	(3,739)	(2,519)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(3,739)	(2,519)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	1,339	1,898
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	1,680	7
Net Amount	(720)	(614)
Futures
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(1,324)	(445)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(1,324)	(445)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	0	0
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	1,324	445
Net Amount	0	0
Loan warehouse debt
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(368,679)	(1,023,740)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(368,679)	(1,023,740)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	368,679	1,023,740
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	(435,496)	(693,641)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(435,496)	(693,641)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	435,496	693,641
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	0	0
Net Amount	$ 0	$ 0

Offsetting Asset and Liabilities [Line Items]

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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 of obligation to return securities 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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Amount, after the effects of master netting arrangements, of derivative liabilities, securities sold under agreements to repurchase and securities loaned. Includes liabilities 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 liabilities, securities sold under agreements to repurchase and securities loaned offset against a right to receive collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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