Derivative Instruments - Schedule of Gain or Loss Recognized on Interest Rate Derivatives Designated as Cash Flow Hedges (Details) - Cash flow hedging - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Interest Rate Cash Flow Hedges [Roll Forward]
Balance at beginning of period	$ (6,921)	$ (10,706)	$ (7,462)	$ (11,706)
Amount of loss recognized in cumulative other comprehensive loss	(274)	(632)	(960)	(861)
Amount of loss reclassified from cumulative other comprehensive loss into interest expense	1,232	1,259	2,459	2,488
Unrealized gain on derivative instruments	958	627	1,499	1,627
Balance at end of period	$ (5,963)	$ (10,079)	$ (5,963)	$ (10,079)