Summary of Significant Accounting Policies - Revenue by Contract Type (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Revenue By Contract Type [Line Items]
Revenue percent	100.00%	100.00%	100.00%
Time-and-material [Member]
Schedule Of Revenue By Contract Type [Line Items]
Revenue percent	84.70%	85.90%	86.50%
Progress Billing [Member]
Schedule Of Revenue By Contract Type [Line Items]
Revenue percent	10.50%	10.80%	10.80%
Percentage-of-completion [Member]
Schedule Of Revenue By Contract Type [Line Items]
Revenue percent	4.80%	3.30%	2.70%