Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 1,549,171,000	$ 351,223,000
Eurodollar Futures Contracts	5,174,000	2,446,000
Interest Rate Swaption	1,216,838
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	1,549,171,000	351,223,000
Eurodollar Futures Contracts	5,174,000	2,446,000
Interest Rate Swaption	1,216,838
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	5,174,000	2,446,000
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	1,549,171,000	351,223,000
Eurodollar Futures Contracts	0	0
Interest Rate Swaption	1,216,838
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