Business Combinations - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) acquisition	Mar. 31, 2017 USD ($) acquisition	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 41,204,000
Number of acquisitions | acquisition	2	0
Aggregate purchase price of acquisitions	$ 21,400
Payments to Acquire Businesses, Gross	33,576,000
Goodwill assigned	31,367,000
Goodwill currently deductible for income tax purposes	26,900,000
Goodwill related to the recorded earn-out payables	4,500,000
Total revenues related to acquisitions	900,000
Income before income taxes related to acquisitions	100,000
Estimated acquisition earn-out payables	40,600,000	$ 57,408,000	$ 36,175,000	$ 63,821,000
Maximum Future Contingency payments Acquisitions	84,200,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 29,100,000
Purchased customer accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years
Non-compete agreements
Business Acquisition [Line Items]
Weighted average life (years)	5 years
Services
Business Acquisition [Line Items]
Goodwill assigned	$ 31,356,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 11,500,000
Business Combinations - Asset Deals [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	2
Other Acquisitions [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 41,204,000
Goodwill assigned	$ 31,367,000