Equity Method Investments Summarized Combined Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investments and Joint Ventures [Abstract]
Current assets	$ 102,908	$ 90,656
Non-current assets	79,364	78,913
Total assets	182,272	169,569
Current liabilities	37,113	32,520
Non-current liabilities	13,751	14,384
Equity	131,408	122,665
Total liabilities and equity	182,272	169,569
Revenues	336,349	320,078	$ 289,994
Operating expenses	289,224	274,952	250,170
Net income	$ 45,648	$ 43,410	$ 37,951