Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	$ 8,189
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	7,743
Payments to Acquire Businesses, Gross	149,992
Recorded Earn-Out Payable	22,071
Fixed assets	290
Goodwill	129,775
Purchased customer accounts	40,713
Non-compete agreements	170
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	1,528
Total assets acquired	188,408
Other current liabilities	(13,959)
Total liabilities assumed	(13,959)
Net assets acquired	174,449
Maximum Potential Earn- Out Payable	38,048
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	0
Fixed assets	23
Goodwill	9,017
Purchased customer accounts	4,912
Non-compete agreements	105
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	14,057
Other current liabilities	(1,504)
Total liabilities assumed	(1,504)
Net assets acquired	12,553
Opus Advisory Group, LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	0
Fixed assets	12
Goodwill	17,938
Purchased customer accounts	5,051
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	23,022
Other current liabilities	0
Total liabilities assumed	0
Net assets acquired	23,022
Kerxton Insurance Agency, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	0
Fixed assets	10
Goodwill	13,417
Purchased customer accounts	4,712
Non-compete agreements	22
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	18,161
Other current liabilities	(1,414)
Total liabilities assumed	(1,414)
Net assets acquired	16,747
Automotive Development Group, LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	0
Fixed assets	67
Goodwill	35,436
Purchased customer accounts	9,136
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	44,660
Other current liabilities	0
Total liabilities assumed	0
Net assets acquired	44,660
Servco Pacific, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	8,189
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	7,743
Fixed assets	178
Goodwill	53,967
Purchased customer accounts	16,902
Non-compete agreements	1
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	1,528
Total assets acquired	88,508
Other current liabilities	(11,041)
Total liabilities assumed	(11,041)
Net assets acquired	77,467
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 5,600