Derivatives and other Hedging Instruments - Additional Information (Details)	3 Months Ended	6 Months Ended	9 Months Ended
	Mar. 31, 2015 USD ($)	Jun. 30, 2015 USD ($) Contract	Sep. 30, 2015 USD ($) Contract	Dec. 31, 2014 Contract	Sep. 30, 2014 Contract	Jun. 30, 2014 Contract	Dec. 31, 2013 Contract
Derivative [Line Items]
Estimated amount related to derivatives reclassified to interest expense during the next 12 months			$ 4,842,000
Early Terminated Interest Rate Swaps
Derivative [Line Items]
Notional Amount		$ 1,400,000,000
Payment on termination of interest rate swaps		$ 5,210,000
Derivative, Maturity Date		Dec. 31, 2015
Interest Rate Swaptions
Derivative [Line Items]
Number of derivative instruments outstanding | Contract			6	4
Proceeds from swaptions termination	$ 2,049,000
Eurodollar Future
Derivative [Line Items]
Notional Amount			$ 1,000,000
Number of derivative instruments outstanding | Contract		131,276	123,796	130,074	131,132	123,275	95,327
Derivative term of contract			3 months
Futures contracts range			2015 through 2022
Fair value of derivatives in net liability position, which includes accrued interest but excludes any adjustment for nonperformance risk, related to agreements			$ 22,964,000
15-year TBA Dollar Roll Securities
Derivative [Line Items]
Derivative term of contract			15 years