Revenue (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	$ 122,454	$ 118,702
Contract with customer, assets	24,862		$ 25,320
Contract with customer, liability	564		$ 1,420
Contract assets transferred to receivables	8,366
Revenue recognized from contract liability	401
Cash proceeds from liability contract	346
Revenue remaining performance obligation	$ 220,301
Over time
Disaggregation of Revenue
Customer revenue transferred over-time (percentage)	25.50%	23.20%
Revenue from contract with customer including assessed tax	$ 31,222	$ 27,573
Point in time
Disaggregation of Revenue
Customer revenue transferred over-time (percentage)	74.50%	76.80%
Revenue from contract with customer including assessed tax	$ 91,232	$ 91,129
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction
Revenue remaining performance obligation (percentage)	2.70%
Performance obligations expected to be satisfied, expected timing
Performance Based | Over time
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	$ 24,561	21,610
Delivery Based | Over time
Disaggregation of Revenue
Revenue from contract with customer including assessed tax	$ 6,661	$ 5,963