Derivative Financial Instruments - Summary of Eurodollar Futures Positions (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	1.73%	2.02%
Notional Amount	$ 760,000,000	$ 257,353,000
Open Equity	(4,302,000)	4,898,000
Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	1.73%	2.02%
Notional Amount	760,000,000	257,353,000
Open Equity	(4,302,000)	4,898,000
Year 2014 Expiration [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	0.00%	0.40%
Notional Amount	0	262,500,000
Open Equity	0	(189,000)
Year 2014 Expiration [Member] | Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	0.00%	0.40%
Notional Amount	0	262,500,000
Open Equity	0	(189,000)
Year 2015 Expiration [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	0.63%	0.80%
Notional Amount	650,000,000	275,000,000
Open Equity	(1,039,000)	(146,000)
Year 2015 Expiration [Member] | Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	0.63%	0.80%
Notional Amount	650,000,000	275,000,000
Open Equity	(1,039,000)	(146,000)
Year 2016 Expiration [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	1.54%	1.90%
Notional Amount	800,000,000	250,000,000
Open Equity	139,000	1,367,000
Year 2016 Expiration [Member] | Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	1.54%	1.90%
Notional Amount	800,000,000	250,000,000
Open Equity	139,000	1,367,000
Year 2017 Expiration [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	2.23%	3.03%
Notional Amount	800,000,000	250,000,000
Open Equity	(1,041,000)	2,291,000
Year 2017 Expiration [Member] | Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	2.23%	3.03%
Notional Amount	800,000,000	250,000,000
Open Equity	(1,041,000)	2,291,000
Year 2018 Expiration [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	2.54%	3.77%
Notional Amount	800,000,000	250,000,000
Open Equity	(2,361,000)	1,575,000
Year 2018 Expiration [Member] | Eurodollar Future [Member]
Derivatives, Fair Value [Line Items]
Locked-In LIBOR Rate	2.54%	3.77%
Notional Amount	800,000,000	250,000,000
Open Equity	$ (2,361,000)	$ 1,575,000