Assets and Liabilities Recorded at Fair Value on Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
[EstimateOfFairValueFairValueDisclosureMember]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	$ 115,380	$ 56,002
Eurodollar Futures Contracts		34
[FairValueInputsLevel1Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Eurodollar Futures Contracts		34
[FairValueInputsLevel2Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	115,380	56,002
Eurodollar Futures Contracts		0
[FairValueInputsLevel3Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage-backed securities	0	0
Eurodollar Futures Contracts		$ 0