Derivative Instruments - Schedule of Gain or Loss Recognized on Interest Rate Derivatives Designated as Cash Flow Hedges (Details) (Cash flow hedging, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flow hedging
Interest Rate Cash Flow Hedges [Roll Forward]
Balance at beginning of period	$ (11,706)	$ (16,624)	$ (15,796)
Amount of loss recognized in cumulative other comprehensive loss	(776)	(103)	(5,790)
Amount of loss reclassified from cumulative other comprehensive loss into interest expense	5,020	5,021	4,962
Unrealized gain (loss) on derivative instruments	4,244	4,918	(828)
Balance at end of period	$ (7,462)	$ (11,706)	$ (16,624)