Organization and Significant Accounting Policies - Additional Information (Detail)	3 Months Ended				12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Bond shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Description of management fee					In exchange for services, our Manager receives a management fee, paid monthly in arrears, in an amount equal to one-twelfth of 1.20% of our Equity (as defined in the Management Agreement).
Monthly management fee payment in arrears, one-twelfth of percentage of equity					1.20%
Gain or loss recorded upon consolidation					$ 0
MBS initial fixed interest rate required, period					three to ten years
Unrealized (loss) on Agency MBS held as trading investments	$ (14,925,000)	$ 1,148,000	$ 0	$ 0	$ (13,800,000)	$ 0
Number of Non-Agency MBS bonds impaired | Bond					6
Impairment charge on Non-Agency MBS	1,548,000	$ 0	$ 0	$ 0	$ 1,548,000	$ 0	$ 0
Building Estimated Value					27 years 6 months
Building Salvage Value	0				$ 0
Unrecognized tax benefits	0				0
Unrecognized tax benefits, accrued interest or penalties	$ 0				$ 0
Options to purchase common stock not included in the computation of diluted EPS | shares					0	0	5,000
Agency MBS
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Unrealized (loss) on Agency MBS held as trading investments					$ (13,777,000)	$ 0	$ 0
Minimum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
MBS initial fixed interest rate required, period					3 years
Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
MBS initial fixed interest rate required, period					10 years