Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ 876,589
Accumulated other comprehensive loss at end of period	891,987
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(69)	$ (133)
Losses reclassified from net accumulated other comprehensive loss	23	19
Net current period other comprehensive income	23	19
Accumulated other comprehensive loss at end of period	$ (46)	$ (114)