Segments - Revenue and Gross Margin by Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue and other financial information by reportable segment
Revenues	$ 232,372	$ 197,853	$ 671,282	$ 585,720
Total gross margin	113,273	87,416	326,649	275,602
Contract Operations
Revenue and other financial information by reportable segment
Total gross margin	100,453	81,573	294,696	256,331
Contract Operations | Contract operations
Revenue and other financial information by reportable segment
Revenues	169,509	153,524	496,156	454,622
Aftermarket Services
Revenue and other financial information by reportable segment
Total gross margin	12,820	5,843	31,953	19,271
Aftermarket Services | Aftermarket services
Revenue and other financial information by reportable segment
Revenues	$ 62,863	$ 44,329	$ 175,126	$ 131,098