Eurodollar Futures Positions (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Eurodollar Futures [Line Items]
Locked-In LIBOR Rate	1.00%
Open Equity	$ (484,000)
Cash Held by Broker as Margin on Eurodollar Futures Contracts	2,031,000	0
[Year2013ExpirationMember]
Eurodollar Futures [Line Items]
Locked-In LIBOR Rate	0.37%
Notional Amount	250,000,000
Open Equity	54,000
[Year2014ExpirationMember]
Eurodollar Futures [Line Items]
Locked-In LIBOR Rate	0.48%
Notional Amount	250,000,000
Open Equity	14,000
[Year2015ExpirationMember]
Eurodollar Futures [Line Items]
Locked-In LIBOR Rate	0.75%
Notional Amount	250,000,000
Open Equity	(98,000)
[Year2016ExpirationMember]
Eurodollar Futures [Line Items]
Locked-In LIBOR Rate	1.29%
Notional Amount	250,000,000
Open Equity	(155,000)
[Year2017ExpirationMember]
Eurodollar Futures [Line Items]
Locked-In LIBOR Rate	1.99%
Notional Amount	250,000,000
Open Equity	$ (299,000)