Acquisitions - Estimated Fair Values of Identifiable Intangible Assets and Related Weighted Average Amortization (Detail) - Acquisitions $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Finite-Lived Intangible Assets [Line Items]
Estimated fair value at acquisition date	$ 103,791
Weighted average amortization period at acquisition dates (in years)	8 years 1 month
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Estimated fair value at acquisition date	$ 76,213
Weighted average amortization period at acquisition dates (in years)	6 years 9 months 18 days
Backlog
Finite-Lived Intangible Assets [Line Items]
Estimated fair value at acquisition date	$ 333
Weighted average amortization period at acquisition dates (in years)	2 years
Trade names
Finite-Lived Intangible Assets [Line Items]
Estimated fair value at acquisition date	$ 18,815
Weighted average amortization period at acquisition dates (in years)	15 years
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Estimated fair value at acquisition date	$ 8,430
Weighted average amortization period at acquisition dates (in years)	5 years