Goodwill and Other Intangible Assets - Other Intangible Assets (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	$ 645,434	$ 598,686
Accumulated Amortization	(364,023)	(335,507)
Intangible Assets, Net	$ 281,411	263,179
Remaining Weighted Average Amortization Period in Years	8 years 3 months 18 days
Indefinite-lived Intangible Assets [Line Items]
Intangible Assets	$ 648,434	598,686
Accumulated Amortization	(364,023)	(335,507)
Intangible Assets, Net	284,411	263,179
Engineering license
Indefinite-lived Intangible Assets [Line Items]
Intangible Assets	3,000	0
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	354,579	327,334
Accumulated Amortization	(158,622)	(137,333)
Intangible Assets, Net	$ 195,957	190,001
Remaining Weighted Average Amortization Period in Years	6 years 4 months 24 days
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (158,622)	(137,333)
Backlog
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	137,238	136,266
Accumulated Amortization	(135,657)	(135,847)
Intangible Assets, Net	$ 1,581	419
Remaining Weighted Average Amortization Period in Years	24 days
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (135,657)	(135,847)
Trade names
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	81,834	74,797
Accumulated Amortization	(20,591)	(17,057)
Intangible Assets, Net	$ 61,243	57,740
Remaining Weighted Average Amortization Period in Years	15 years 7 months 6 days
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (20,591)	(17,057)
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	40,267	37,760
Accumulated Amortization	(29,691)	(27,659)
Intangible Assets, Net	$ 10,576	10,101
Remaining Weighted Average Amortization Period in Years	3 years 8 months 12 days
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (29,691)	(27,659)
Patented rights and developed technology
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	22,516	22,529
Accumulated Amortization	(18,838)	(17,611)
Intangible Assets, Net	$ 3,678	4,918
Remaining Weighted Average Amortization Period in Years	2 years 10 months 24 days
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (18,838)	(17,611)
Curriculum
Finite-Lived Intangible Assets [Line Items]
Intangible Assets	9,000	0
Accumulated Amortization	(624)	0
Intangible Assets, Net	$ 8,376	0
Remaining Weighted Average Amortization Period in Years	9 years 3 months 18 days
Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (624)	$ 0