Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	$ 11,303
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	13,407
Payments to Acquire Businesses, Gross	265,890
Recorded Earn-Out Payable	51,717
Fixed assets	832
Goodwill	214,559
Purchased customer accounts	101,781
Non-compete agreements	506
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	762
Total assets acquired	343,150
Other current liabilities	(21,639)
Total liabilities assumed	(22,204)
Net assets acquired	320,946
Maximum Potential Earn- Out Payable	97,895
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	253
Fixed assets	99
Goodwill	20,245
Purchased customer accounts	12,653
Non-compete agreements	273
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	33,523
Other current liabilities	(1,715)
Total liabilities assumed	(1,715)
Net assets acquired	31,808
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	11
Goodwill	17,944
Purchased customer accounts	5,008
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	22,984
Other current liabilities	0
Total liabilities assumed	0
Net assets acquired	22,984
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	1,500
Fixed assets	67
Goodwill	36,236
Purchased customer accounts	9,751
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	47,575
Other current liabilities	0
Total liabilities assumed	0
Net assets acquired	47,575
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	8,075
Fixed assets	178
Goodwill	53,901
Purchased customer accounts	16,902
Non-compete agreements	1
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	1,528
Total assets acquired	88,774
Other current liabilities	(11,307)
Total liabilities assumed	(11,307)
Net assets acquired	77,467
National Programs [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	18,700
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	5,500
Services [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 17,300