Mortgage-Backed Securities - Gross Unrealized Gains (Losses) (Detail) - Residential Mortgage-Backed Securities - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	$ 911,158	$ 844,273
Gross Unrealized Gain Included in Accumulated Deficit	33,900	23,240
Total Gross Unrealized Gain	945,058	867,512
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(3,968)	(70,752)
Gross Unrealized Loss Included in Accumulated Deficit	(60,212)	(71,738)
Total Gross Unrealized Loss	(64,180)	(142,490)
Non-Agency RMBS - Senior
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	728,950	736,040
Total Gross Unrealized Gain	728,950	736,040
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(3,228)	(8,779)
Total Gross Unrealized Loss	(3,228)	(8,779)
Non-Agency RMBS - Senior interest-only
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Deficit	28,130	18,113
Total Gross Unrealized Gain	28,130	18,113
Gross Unrealized Loss Included in Accumulated Deficit	(42,356)	(52,457)
Total Gross Unrealized Loss	(42,356)	(52,457)
Non-Agency RMBS - Subordinated
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	76,632	81,588
Gross Unrealized Gain Included in Accumulated Deficit	3,367	2,309
Total Gross Unrealized Gain	79,999	83,896
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(90)	(1,971)
Gross Unrealized Loss Included in Accumulated Deficit	(7,370)	(5,606)
Total Gross Unrealized Loss	(7,460)	(7,577)
Non-Agency RMBS - Subordinated interest-only
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Deficit	83	62
Total Gross Unrealized Gain	83	62
Gross Unrealized Loss Included in Accumulated Deficit	(2,057)	(4,035)
Total Gross Unrealized Loss	(2,057)	(4,035)
Agency MBS - Residential
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	53,860	18,593
Total Gross Unrealized Gain	53,860	18,593
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(294)	(52,001)
Total Gross Unrealized Loss	(294)	(52,001)
Agency MBS - Commercial
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	51,716	8,052
Total Gross Unrealized Gain	51,716	8,052
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(356)	(8,001)
Total Gross Unrealized Loss	(356)	(8,001)
Agency MBS - Interest-only
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Deficit	2,320	2,756
Total Gross Unrealized Gain	2,320	2,756
Gross Unrealized Loss Included in Accumulated Deficit	(8,429)	(9,640)
Total Gross Unrealized Loss	$ (8,429)	$ (9,640)