Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	$ 3,218
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	(4,345)
Payments to Acquire Businesses, Gross	98,299
Recorded Earn-Out Payable	18,155
Fixed assets	112
Goodwill	80,432
Purchased customer accounts	35,782
Non-compete agreements	138
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	316
Total assets acquired	115,653
Other current liabilities	1,438
Total liabilities assumed	1,409
Net assets acquired	117,062
Maximum Potential Earn- Out Payable	27,521
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	(6,835)
Fixed assets	(121)
Goodwill	15,911
Purchased customer accounts	6,298
Non-compete agreements	64
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	302
Total assets acquired	15,619
Other current liabilities	5,734
Total liabilities assumed	5,734
Net assets acquired	21,353
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	588
Fixed assets	40
Goodwill	16,031
Purchased customer accounts	6,500
Non-compete agreements	41
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	23,200
Other current liabilities	(367)
Total liabilities assumed	(367)
Net assets acquired	22,833
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	194
Fixed assets	114
Goodwill	18,146
Purchased customer accounts	11,360
Non-compete agreements	11
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	29,825
Other current liabilities	(3,449)
Total liabilities assumed	(3,449)
Net assets acquired	26,376
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	29
Goodwill	11,336
Purchased customer accounts	4,324
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	15,710
Other current liabilities	0
Total liabilities assumed	0
Net assets acquired	15,710
Servco Pacific, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	3,218
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	1,708
Fixed assets	50
Goodwill	19,008
Purchased customer accounts	7,300
Non-compete agreements	1
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	14
Total assets acquired	31,299
Other current liabilities	(480)
Total liabilities assumed	(509)
Net assets acquired	30,790
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	6,100
Services [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	19,200
Services [Member] | Servco Pacific, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Payments to Acquire Businesses, Gross	29,106
Recorded Earn-Out Payable	1,684
Net assets acquired	30,790
Maximum Potential Earn- Out Payable	$ 2,500