Derivative Instruments - TBA Commitments (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Asset	$ 91,000	$ 763,000
Fair Value, Liability	(3,431,000)	(4,833,000)
TBA Commitments
Notional Amount: Net Purchase Commitment	750,000,000	1,265,000,000
Contractual Forward Price	783,346,000	1,296,369,000
Market Price	781,258,000	1,296,978,000
Fair Value, Asset	0	763,000
Fair Value, Liability	(2,088,000)	(154,000)
Fair Value	(2,088,000)	609,000
TBA Commitments | Dollar Roll Positions Four Point Five Percent Thirty Year Mortgage Backed Securities Purchase (Sale) Commitments, Purchase
Notional Amount: Net Purchase Commitment	250,000,000
Contractual Forward Price	258,549,000
Market Price	257,945,000
Fair Value, Liability	(604,000)
TBA Commitments | Dollar Roll Positions Five Point Zero Percent Thirty Year Mortgage Backed Securities Purchase (Sale) Commitments, Purchase
Notional Amount: Net Purchase Commitment	500,000,000
Contractual Forward Price	524,797,000
Market Price	523,313,000
Fair Value, Liability	$ (1,484,000)
TBA Commitments | Dollar Roll Positions Three Point Zero Percent Fiften Year Mortgage Backed Securities Purchase (Sale) Commitments, Purchase
Notional Amount: Net Purchase Commitment		250,000,000
Contractual Forward Price		254,873,000
Market Price		254,766,000
Fair Value, Liability		(107,000)
TBA Commitments | Dollar Roll Positions Three Point Five Percent Thirty Year Mortgage Backed Securities Purchase (Sale) Commitments, Purchase
Notional Amount: Net Purchase Commitment		1,015,000,000
Contractual Forward Price		1,041,496,000
Market Price		1,042,212,000
Fair Value, Asset		$ 716,000