Mortgage-Backed Securities - Unrealized Loss Positions (Detail) - Residential Mortgage-Backed Securities $ in Thousands	Mar. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 3,594,143	$ 3,815,885
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (109,283)	$ (110,548)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	778	846
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 237,075	$ 252,257
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (61,779)	$ (59,127)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	159	156
Estimated Fair Value of RMBS in continuous loss position	$ 3,831,218	$ 4,068,142
Unrealized Losses on RMBS in continuous loss position	$ (171,062)	$ (169,675)
Number of Securities in continuous loss position | security	937	1,002
Non-Agency RMBS - Senior
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 9,776	$ 12,384
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (281)	$ (250)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	2	3
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 0	$ 0
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ 0	$ 0
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	0	0
Estimated Fair Value of RMBS in continuous loss position	$ 9,776	$ 12,384
Unrealized Losses on RMBS in continuous loss position	$ (281)	$ (250)
Number of Securities in continuous loss position | security	2	3
Non-Agency RMBS - Senior interest-only
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 82,622	$ 96,399
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (16,387)	$ (13,600)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	59	62
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 76,566	$ 78,516
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (47,417)	$ (43,931)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	91	86
Estimated Fair Value of RMBS in continuous loss position	$ 159,188	$ 174,915
Unrealized Losses on RMBS in continuous loss position	$ (63,804)	$ (57,531)
Number of Securities in continuous loss position | security	150	148
Non-Agency RMBS - Subordinated
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 27,875	$ 56,015
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (776)	$ (412)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	4	7
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 3,160	$ 2,826
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (917)	$ (824)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	4	4
Estimated Fair Value of RMBS in continuous loss position	$ 31,035	$ 58,841
Unrealized Losses on RMBS in continuous loss position	$ (1,693)	$ (1,236)
Number of Securities in continuous loss position | security	8	11
Non-Agency RMBS - Subordinated interest-only
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 609	$ 748
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (345)	$ (230)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	2	2
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 4,963	$ 11,276
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (1,587)	$ (1,624)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	2	3
Estimated Fair Value of RMBS in continuous loss position	$ 5,572	$ 12,024
Unrealized Losses on RMBS in continuous loss position	$ (1,932)	$ (1,854)
Number of Securities in continuous loss position | security	4	5
Agency MBS - Residential
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 2,243,923	$ 2,338,910
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (45,850)	$ (48,084)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	109	106
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 53,638	$ 54,943
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (1,609)	$ (1,615)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	1	1
Estimated Fair Value of RMBS in continuous loss position	$ 2,297,561	$ 2,393,853
Unrealized Losses on RMBS in continuous loss position	$ (47,459)	$ (49,699)
Number of Securities in continuous loss position | security	110	107
Agency MBS - Commercial
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 1,164,212	$ 1,247,923
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (43,044)	$ (45,802)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	580	646
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 51,932	$ 51,733
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (3,848)	$ (4,035)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	46	46
Estimated Fair Value of RMBS in continuous loss position	$ 1,216,144	$ 1,299,656
Unrealized Losses on RMBS in continuous loss position	$ (46,892)	$ (49,837)
Number of Securities in continuous loss position | security	626	692
Agency MBS - Interest-only
Investment Holdings [Line Items]
Estimated Fair Value of RMBS in continuous loss position for less than 12 consecutive months	$ 65,126	$ 63,506
Unrealized Losses on RMBS in continuous loss position for less than 12 consecutive months	$ (2,600)	$ (2,170)
Number of Securities of RMBS in continuous loss position for less than 12 months | security	22	20
Estimated Fair Value of RMBS in continuous loss position for 12 or more consecutive months	$ 46,816	$ 52,963
Unrealized Losses on RMBS in continuous loss position for 12 or more consecutive months	$ (6,401)	$ (7,098)
Number of Securities on RMBS in continuous loss position for 12 or more consecutive months | security	15	16
Estimated Fair Value of RMBS in continuous loss position	$ 111,942	$ 116,469
Unrealized Losses on RMBS in continuous loss position	$ (9,001)	$ (9,268)
Number of Securities in continuous loss position | security	37	36