Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Eurodollar Futures Contracts	$ 2,031,000	$ 0
[EstimateOfFairValueFairValueDisclosureMember]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	360,260,000	115,380,000
Eurodollar Futures Contracts	2,031,000	0
[FairValueInputsLevel1Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Eurodollar Futures Contracts	2,031,000	0
[FairValueInputsLevel2Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	360,260,000	115,380,000
Eurodollar Futures Contracts	0	0
[FairValueInputsLevel3Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Eurodollar Futures Contracts	$ 0	$ 0