Mortgage-Backed Securities - Gross Unrealized Gains (Losses) (Detail) - Residential Mortgage-Backed Securities - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	$ 822,418	$ 818,127
Gross Unrealized Gain Included in Cumulative Earnings	21,242	23,840
Total Gross Unrealized Gain	843,660	841,967
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(94,707)	(100,021)
Gross Unrealized Loss Included in Cumulative Earnings	(76,355)	(69,654)
Total Gross Unrealized Loss	(171,062)	(169,675)
Non-Agency RMBS - Senior
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	739,946	732,133
Total Gross Unrealized Gain	739,946	732,133
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(281)	(250)
Total Gross Unrealized Loss	(281)	(250)
Non-Agency RMBS - Senior interest-only
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Cumulative Earnings	16,095	18,674
Total Gross Unrealized Gain	16,095	18,674
Gross Unrealized Loss Included in Cumulative Earnings	(63,804)	(57,531)
Total Gross Unrealized Loss	(63,804)	(57,531)
Non-Agency RMBS - Subordinated
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	70,572	74,584
Gross Unrealized Gain Included in Cumulative Earnings	4,167	3,273
Total Gross Unrealized Gain	74,739	77,857
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(75)	(235)
Gross Unrealized Loss Included in Cumulative Earnings	(1,618)	(1,001)
Total Gross Unrealized Loss	(1,693)	(1,236)
Non-Agency RMBS - Subordinated interest-only
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Cumulative Earnings	103	0
Total Gross Unrealized Gain	103	0
Gross Unrealized Loss Included in Cumulative Earnings	(1,932)	(1,854)
Total Gross Unrealized Loss	(1,932)	(1,854)
Agency MBS - Residential
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	10,566	11,235
Total Gross Unrealized Gain	10,566	11,235
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(47,459)	(49,699)
Total Gross Unrealized Loss	(47,459)	(49,699)
Agency MBS - Commercial
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Accumulated Other Comprehensive Income	1,334	175
Total Gross Unrealized Gain	1,334	175
Gross Unrealized Loss Included in Accumulated Other Comprehensive Income	(46,892)	(49,837)
Total Gross Unrealized Loss	(46,892)	(49,837)
Agency MBS - Interest-only
Investment Holdings [Line Items]
Gross Unrealized Gain Included in Cumulative Earnings	877	1,893
Total Gross Unrealized Gain	877	1,893
Gross Unrealized Loss Included in Cumulative Earnings	(9,001)	(9,268)
Total Gross Unrealized Loss	$ (9,001)	$ (9,268)