Debt Obligations - Repurchase Obligations Outstanding (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 3,279,082,000
Collateral Assets	2,606,316,000
Potential	2,011,612,000
Repurchase Borrowings Outstanding	1,779,650,000	1,109,353,000
Available	231,962,000
Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	2,936,572,000
Collateral Assets	2,281,872,000
Potential	1,755,719,000
Repurchase Borrowings Outstanding	1,523,757,000	863,622,000
Available	231,962,000
Bank of America [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	517,280,000
Potential	406,653,000
Repurchase Borrowings Outstanding	387,653,000	271,320,000
Available	19,000,000
Citibank [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	611,459,000
Potential	461,556,000
Repurchase Borrowings Outstanding	351,245,000	334,692,000
Available	110,311,000
JP Morgan [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	510,697,000
Collateral Assets	467,722,000
Potential	354,776,000
Repurchase Borrowings Outstanding	293,600,000	257,610,000
Available	61,176,000
Wells Fargo [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	301,083,000
Potential	231,600,000
Repurchase Borrowings Outstanding	190,125,000
Available	41,475,000
Wells Fargo [Member] | Asset-Specific Repurchase Agreements [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	148,110,000
Collateral Assets	155,184,000
Potential	120,485,000
Repurchase Borrowings Outstanding	120,485,000	245,731,000
Morgan Stanley [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	425,875,000
Collateral Assets	169,804,000
Potential	135,765,000
Repurchase Borrowings Outstanding	135,765,000
MetLife [Member] | Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	214,524,000
Potential	165,369,000
Repurchase Borrowings Outstanding	165,369,000
Goldman Sachs [Member] | Asset-Specific Repurchase Agreements [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	194,400,000
Collateral Assets	169,260,000
Potential	135,408,000
Repurchase Borrowings Outstanding	$ 135,408,000