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Agency MBS and Non-Agency MBS, which are Carried at Fair Value (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]    
Fair value $ 779,995 $ 760,825
Agency MBS    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 3,875,242 4,273,331
Paydowns receivable 16,474 13,873
Unrealized gains 20,071 27,945
Unrealized losses (77,443) (36,352)
Available-for-sale Securities, Total 3,834,344 4,278,797
Agency MBS | 15-Year Fixed-Rate    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 1,167,834 1,277,570
Unrealized gains 134 595
Unrealized losses (35,902) (16,338)
Available-for-sale Securities, Total 1,132,066 1,261,827
Agency MBS | 20-Year and 30-Year Fixed Rate Debt    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 878,941 882,633
Unrealized gains 2,044 3,092
Unrealized losses (18,925) (5,298)
Available-for-sale Securities, Total 862,060 880,427
Agency MBS | Freddie Mac    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 1,452,764 1,678,835
Paydowns receivable 16,474 13,873
Unrealized gains 4,463 6,597
Unrealized losses (32,606) (18,205)
Available-for-sale Securities, Total 1,441,095 1,681,100
Agency MBS | Fannie Mae    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 2,422,478 2,594,496
Unrealized gains 15,608 21,348
Unrealized losses (44,837) (18,147)
Available-for-sale Securities, Total 2,393,249 2,597,697
Agency MBS | ARMs    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 997,383 1,249,366
Paydowns receivable 7,611 5,943
Unrealized gains 17,647 23,425
Unrealized losses (1,217) (1,871)
Available-for-sale Securities, Total 1,021,424 1,276,863
Agency MBS | Hybrids    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 831,084 863,762
Paydowns receivable 8,863 7,930
Unrealized gains 246 833
Unrealized losses (21,399) (12,845)
Available-for-sale Securities, Total 818,794 859,680
MBS    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 4,625,858 5,003,955
Paydowns receivable 16,502 13,873
Unrealized gains 53,077 61,300
Unrealized losses (81,098) (39,506)
Available-for-sale Securities, Total 4,614,339 5,039,622
Non-Agency MBS    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 750,616 730,624
Paydowns receivable 28  
Unrealized gains 33,006 33,355
Unrealized losses (3,655) (3,154)
Fair value $ 779,995 $ 760,825