Revenue and Contract Costs - Pass Through Costs (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Contractors [Abstract]
Pass-through costs as a percentage of revenues	48.40%	33.80%	45.30%	35.00%