Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 2,117,609,000	$ 1,549,171,000
Eurodollar Futures Contracts	9,188,000	5,174,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	2,117,609,000	1,549,171,000
Eurodollar Futures Contracts	9,188,000	5,174,000
Interest Rate Swaption		1,217,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Eurodollar Futures Contracts	9,188,000	5,174,000
Interest Rate Swaption		0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	2,117,609,000	1,549,171,000
Eurodollar Futures Contracts	0	0
Interest Rate Swaption		1,217,000
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Eurodollar Futures Contracts	$ 0	0
Interest Rate Swaption		$ 0