Segments - Revenue and Gross Margin by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue and other financial information by reportable segment
Revenues	$ 212,040	$ 189,885
Total gross margin	105,108	92,056
Contract Operations
Revenue and other financial information by reportable segment
Revenues	161,197	149,984
Total gross margin	96,602	85,887
Aftermarket Services
Revenue and other financial information by reportable segment
Revenues	50,843	39,901
Total gross margin	$ 8,506	$ 6,169