Revenue (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Revenue remaining performance obligation	$ 251,617		$ 251,617
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	167,094	$ 131,492	462,438	$ 395,054
Contract with customer, assets	27,796		27,796		$ 25,320
Contract with customer, liability	$ 2,423		2,423		$ 1,420
Contract assets transferred to receivables			14,528
Revenue recognized from contract liability			1,146
Cash proceeds from liability contract			$ 1,754
Over time
Disaggregation of Revenue
Customer revenue transferred over-time (percentage)	26.80%	24.00%	25.20%	23.30%
Revenue from contract with customer including assessed tax	$ 44,735	$ 31,543	$ 116,433	$ 92,000
Over time | Performance Based
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	33,225	20,447	87,369	61,892
Over time | Delivery Based
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	$ 11,510	$ 11,096	$ 29,064	$ 30,108
Point in time
Disaggregation of Revenue
Customer revenue transferred over-time (percentage)	73.20%	76.00%	74.80%	76.70%
Revenue from contract with customer including assessed tax	$ 122,359	$ 99,949	$ 346,005	$ 303,054
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Revenue remaining performance obligation (percentage)	10.00%		10.00%
Performance obligations expected to be satisfied, expected timing			1 year