Summary of Significant Accounting Policies - Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Information [Abstract]
Interest paid	$ 7,307	$ 6,877	$ 26,501
Taxes paid	3,125	9,778	1,150
Non-cash activities:
Purchases of property and equipment not paid	$ 2,104	$ 3,241	$ 1,549