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Borrowings Under Repurchase Agreements - Summary of Outstanding Balances, Maximum and Average Balances of Borrowings (Detail) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding $ 735,437  
Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding 735,437 $ 622,194
Wells Fargo [Member] | Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding 0 20,166
Maximum Month-End Balance 22,254  
Average Month-End Balance 4,242  
UBS [Member] | Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding 133,899 133,899
Maximum Month-End Balance 133,899  
Average Month-End Balance 133,899  
DB Facility [Member] | Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding 300,005 300,005
Maximum Month-End Balance 300,005  
Average Month-End Balance 300,005  
JP Morgan Chase [Member] | Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding 253,481 168,124
Maximum Month-End Balance 395,572  
Average Month-End Balance 244,678  
Goldman Sachs [Member] | Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of credit, amount outstanding 48,052 $ 0
Maximum Month-End Balance 52,524  
Average Month-End Balance $ 45,259