Summary of Significant Accounting Policies - Revenue by Contract Type (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Revenue by External Customer [Line Items]
Revenue percent	100.00%	100.00%	100.00%
Time-and-material [Member]
Schedule of Revenue by External Customer [Line Items]
Revenue percent	85.90%	86.50%	88.60%
Progress Billing [Member]
Schedule of Revenue by External Customer [Line Items]
Revenue percent	10.80%	10.80%	9.50%
Percentage-of-completion [Member]
Schedule of Revenue by External Customer [Line Items]
Revenue percent	3.30%	2.70%	1.90%