Revenue and Contract Costs - Pass Through Costs (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Contractors [Abstract]
Pass-through costs as a percentage of revenues	53.10%	35.10%	41.90%	37.60%