Derivatives - Derivative Gain (Loss) Recognized in Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Effect of derivative instruments on results of operations
Interest expense	$ 23,337	$ 22,504	$ 45,884	$ 43,925
Accumulated Other Comprehensive Income (Loss) | Reclassification adjustments
Effect of derivative instruments on results of operations
Interest expense	$ 207		$ 153