Secured Financings - Revolving Repurchase Facilities Outstanding (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 2,904,571,000
Collateral Assets	2,725,187,000
Potential	2,103,144,000
Repurchase Borrowings Outstanding	1,669,406,000	863,622,000
Available	433,738,000
Wells Fargo [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	574,395,000
Potential	447,994,000
Repurchase Borrowings Outstanding	360,725,000
Available	87,269,000
Bank of America [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	537,159,000
Potential	424,404,000
Repurchase Borrowings Outstanding	353,542,000	271,320,000
Available	70,862,000
Citibank [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	584,020,000
Potential	441,567,000
Repurchase Borrowings Outstanding	324,429,000	334,692,000
Available	117,138,000
JP Morgan [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	498,546,000
Collateral Assets	519,159,000
Potential	396,587,000
Repurchase Borrowings Outstanding	269,618,000	257,610,000
Available	126,969,000
MetLife [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000
Collateral Assets	341,524,000
Potential	263,889,000
Repurchase Borrowings Outstanding	232,389,000
Available	31,500,000
Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	406,025,000
Collateral Assets	168,930,000
Potential	128,703,000
Repurchase Borrowings Outstanding	$ 128,703,000