Revenue (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Revenue remaining performance obligation	$ 231,252		$ 231,252
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	172,890	$ 144,860	295,344	$ 263,562
Contract with customer, assets	29,247		29,247		$ 25,320
Contract with customer, liability	3,035		3,035		$ 1,420
Contract assets transferred to receivables			10,872
Revenue recognized from contract liability			740
Cash proceeds from liability contract			$ 2,488
Performance Based
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	$ 29,583
Over time
Disaggregation of Revenue
Customer revenue transferred over-time (percentage)	23.40%	22.70%	24.30%	22.90%
Revenue from contract with customer including assessed tax	$ 40,476	$ 32,884	$ 71,698	$ 60,457
Over time | Performance Based
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	29,583	19,835	54,144	41,445
Over time | Delivery Based
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	$ 10,893	$ 13,049	$ 17,554	$ 19,012
Point in time
Disaggregation of Revenue
Customer revenue transferred over-time (percentage)	76.60%	77.30%	75.70%	77.10%
Revenue from contract with customer including assessed tax	$ 132,414	$ 111,976	$ 223,646	$ 203,105
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Revenue remaining performance obligation (percentage)	11.70%		11.70%
Performance obligations expected to be satisfied, expected timing			1 year