Acquisitions - Estimated Fair Values of Identifiable Intangible Assets and Related Weighted Average Amortization (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date		$ 52,400
Acquisitions 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date	$ 52,364
Weighted average amortization period at acquisition date	7 years 4 months 24 days
Customer relationships | Acquisitions 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date	$ 30,767
Weighted average amortization period at acquisition date	5 years 3 months 18 days
Backlog | Acquisitions 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date	$ 2,158
Weighted average amortization period at acquisition date	1 year
Trade names | Acquisitions 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date	$ 7,689
Weighted average amortization period at acquisition date	15 years
Non-compete agreements | Acquisitions 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date	$ 2,750
Weighted average amortization period at acquisition date	5 years
Curriculum | Acquisitions 2018
Business Acquisition [Line Items]
Estimated fair value at acquisition date	$ 9,000
Weighted average amortization period at acquisition date	10 years