Revenue Recognition - Disaggregation of Revenue (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 302,780
Fixed-Price [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	121,042
Time-and-Materials [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	125,630
Cost-Based [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	56,108
U S Federal Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	133,544
U.S. State and Local Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	31,986
International Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	28,804
Total Government
Disaggregation Of Revenue [Line Items]
Revenue from clients	194,334
Commercial
Disaggregation Of Revenue [Line Items]
Revenue from clients	108,446
Energy, Environmental and Infrastructure [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	124,352
Health, Education and Social Programs [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	123,204
Safety and Security [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	25,322
Consumer and Financial [Member]
Disaggregation Of Revenue [Line Items]
Revenue from clients	$ 29,902