Organization and Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Description of management fee		In exchange for services provided, 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		one to ten years
Unrealized gain on Agency MBS held as trading investments	$ 4,100,000	$ 4,200,000
Number of Non-Agency MBS bonds impaired | item		2
Impairment charge on Non-Agency MBS	905,000	$ 1,637,000
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
Agency MBS
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Cumulative unrealized losses on Agency MBS held as trading investments	10,400,000	10,400,000	$ 13,800,000
Unrealized gain on Agency MBS held as trading investments	$ 4,101,000	$ 4,222,000
Minimum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
MBS initial fixed interest rate required, period		1 year
Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
MBS initial fixed interest rate required, period		10 years