Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Fair Value Of Assets And Liabilities Statement [Line Items]
Payments to Acquire Businesses, Gross	$ 33,576
Recorded Earn-Out Payable	4,524
Fixed assets	21
Goodwill	31,367
Purchased customer accounts	9,774
Non-compete agreements	42
Total assets acquired	41,204
Other current liabilities	(1,414)
Total liabilities assumed	(1,414)
Net assets acquired	39,790
Maximum Potential Earn- Out Payable	6,520
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets	21
Goodwill	31,367
Purchased customer accounts	9,774
Non-compete agreements	42
Total assets acquired	41,204
Other current liabilities	(1,414)
Total liabilities assumed	(1,414)
Net assets acquired	$ 39,790