Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 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	(3,164)
Payments to Acquire Businesses, Gross	149,863
Recorded earn-out payable	23,966
Fixed assets	238
Goodwill	120,502
Purchased customer accounts	54,119
Non-compete agreements	203
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	54
Total assets acquired	175,170
Other current liabilities	847
Total liabilities assumed	818
Net assets acquired	175,988
Maximum potential earn- out payable	43,453
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	(6,658)
Fixed assets	(97)
Goodwill	22,111
Purchased customer accounts	9,013
Non-compete agreements	106
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	(732)
Total assets acquired	23,743
Other current liabilities	5,577
Total liabilities assumed	5,577
Net assets acquired	29,320
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	473
Fixed assets	39
Goodwill	16,249
Purchased customer accounts	6,500
Non-compete agreements	41
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	23,302
Other current liabilities	(469)
Total liabilities assumed	(469)
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	819
Fixed assets	112
Goodwill	16,765
Purchased customer accounts	11,360
Non-compete agreements	11
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	772
Total assets acquired	29,839
Other current liabilities	(3,463)
Total liabilities assumed	(3,463)
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	17
Fixed assets	29
Goodwill	11,319
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	19,108
Purchased customer accounts	7,300
Non-compete agreements	1
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	14
Total assets acquired	31,399
Other current liabilities	(480)
Total liabilities assumed	(509)
Net assets acquired	30,890
United Development Systems, 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	477
Fixed assets	20
Goodwill	15,111
Purchased customer accounts	7,065
Non-compete agreements	11
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	22,684
Other current liabilities	(41)
Total liabilities assumed	(41)
Net assets acquired	22,643
Twinbrook 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	85
Goodwill	19,839
Purchased customer accounts	8,557
Non-compete agreements	12
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	28,493
Other current liabilities	(277)
Total liabilities assumed	(277)
Net assets acquired	28,216
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	19,300
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	30,890
Maximum potential earn- out payable	$ 2,500