Summary of Significant Accounting Policies - Offsetting of Financial Assets, Liabilities, and Collateral (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	$ 20,436	$ 15,257
Gross Amounts Offset in Consolidated Balance Sheet		0
Net Amounts of Assets Presented in Consolidated Balance Sheet	20,436	15,257
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(8,294)	(6,975)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(6,385)	(4,735)
Net Amount	5,757	3,547
Gross Amounts of Recognized Liabilities	(1,416,770)	(1,851,876)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(1,416,770)	(1,851,876)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	1,375,356	1,800,800
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	40,618	50,100
Net Amount	(796)	(976)
Interest rate agreements
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	9,017	7,006
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	9,017	7,006
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(3,555)	(1,160)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(4,822)	(4,360)
Net Amount	640	1,486
Gross Amounts of Recognized Liabilities	(43,982)	(48,173)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(43,982)	(48,173)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	3,555	1,160
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	40,018	47,013
Net Amount	(409)	0
Credit Default Index Swaps
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	3,792	1,598
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	3,792	1,598
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	0	0
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	0	(375)
Net Amount	3,792	1,223
TBAs
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Assets	7,627	6,653
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Assets Presented in Consolidated Balance Sheet	7,627	6,653
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	(4,739)	(5,815)
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Received	(1,563)	0
Net Amount	1,325	838
Gross Amounts of Recognized Liabilities	(5,466)	(9,506)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(5,466)	(9,506)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	4,739	5,815
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	340	2,715
Net Amount	(387)	(976)
Futures
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(260)	(372)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(260)	(372)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	0	0
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	260	372
Net Amount	0	0
Loan warehouse debt
Offsetting Asset and Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	(873,673)	(1,185,316)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(873,673)	(1,185,316)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	873,673	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	(493,389)	(608,509)
Gross Amounts Offset in Consolidated Balance Sheet	0	0
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	(493,389)	(608,509)
Gross Amounts Not Offset in Consolidated Balance Sheet, Financial Instruments	493,389	608,509
Gross Amounts Not Offset in Consolidated Balance Sheet, Cash Collateral Pledged	0	0
Net Amount	$ 0	$ 0