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Agency MBS and Non-Agency MBS, which are Carried at Fair Value (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]    
Fair value $ 690,654 $ 641,246
Agency MBS    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 3,880,675 3,882,611
Paydowns receivable 23,404 31,130
Unrealized gains 39,127 46,724
Unrealized losses (26,152) (35,272)
Available-for-sale Securities, Total 3,917,054 3,925,193
Agency MBS | Hybrids    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 1,002,650 1,106,680
Paydowns receivable 11,542 14,966
Unrealized gains 1,438 1,096
Unrealized losses (8,738) (12,156)
Available-for-sale Securities, Total 1,006,892 1,110,586
Agency MBS | 15-Year Fixed-Rate    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 988,478 872,741
Unrealized gains 1,894 1,142
Unrealized losses (13,827) (19,927)
Available-for-sale Securities, Total 976,545 853,956
Agency MBS | 20-Year and 30-Year Fixed-Rate    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 468,621 140,640
Unrealized gains 4,073 4,420
Unrealized losses (2,050) (27)
Available-for-sale Securities, Total 470,644 145,033
Agency MBS | Freddie Mac    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 1,704,059 1,709,839
Paydowns receivable 23,404 31,130
Unrealized gains 8,987 10,652
Unrealized losses (16,891) (22,301)
Available-for-sale Securities, Total 1,719,559 1,729,320
Agency MBS | Fannie Mae    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 2,176,616 2,164,171
Unrealized gains 30,140 36,069
Unrealized losses (9,261) (12,841)
Available-for-sale Securities, Total 2,197,495 2,187,399
Agency MBS | ARMs    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 1,420,926 1,762,550
Paydowns receivable 11,862 16,164
Unrealized gains 31,722 40,066
Unrealized losses (1,537) (3,162)
Available-for-sale Securities, Total 1,462,973 1,815,618
Agency MBS | Ginnie Mae    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost   8,601
Unrealized gains   3
Unrealized losses   (130)
Available-for-sale Securities, Total   8,474
MBS    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 4,551,217 4,522,314
Paydowns receivable 23,404 31,136
Unrealized gains 64,708 58,723
Unrealized losses (31,621) (45,734)
Available-for-sale Securities, Total 4,607,708 4,566,439
Non-Agency MBS    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 670,542 639,703
Paydowns receivable   6
Unrealized gains 25,581 11,999
Unrealized losses (5,469) (10,462)
Available-for-sale Securities, Total 690,654  
Fair value $ 690,654 $ 641,246