Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019
Business Combination, Separately Recognized Transactions [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)
Cash paid	149,863
Other payable	2,159
Recorded earn-out payable	23,966
Net assets acquired	175,988
Maximum potential earn- out payable	43,453
Fixed assets	238
Goodwill	120,502
Purchased customer accounts	54,119
Non-compete agreements	203
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	54
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	175,170
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	$ 847
Document Period End Date	Jun. 30, 2019
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	$ (29)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(818)
Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	19,300
Services
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	95,100
Wholesale Brokerage
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	6,100
Opus Advisory Group, LLC [Member]
Business Combination, Separately Recognized Transactions [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
Net assets acquired	22,833
Fixed assets	39
Goodwill	16,249
Purchased customer accounts	6,500
Non-compete agreements	41
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	23,302
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(469)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	469
Opus Advisory Group, LLC [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	20,129
Other payable	0
Recorded earn-out payable		$ 2,704
Net assets acquired	22,833
Maximum potential earn- out payable	4,550
Kerxton Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [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
Net assets acquired	26,376
Fixed assets	112
Goodwill	16,765
Purchased customer accounts	11,360
Non-compete agreements	11
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	772
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	29,839
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(3,463)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	3,463
Kerxton Insurance Agency, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	16,420
Other payable	135
Recorded earn-out payable	9,821
Net assets acquired	26,376
Maximum potential earn- out payable	12,996
Automotive Development Group, LLC [Member]
Business Combination, Separately Recognized Transactions [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
Net assets acquired	15,710
Fixed assets	29
Goodwill	11,319
Purchased customer accounts	4,324
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	15,710
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Automotive Development Group, LLC [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	13,990
Other payable	10
Recorded earn-out payable	1,710
Net assets acquired	15,710
Maximum potential earn- out payable	2,000
Servco Pacific, Inc. [Member]
Business Combination, Separately Recognized Transactions [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
Net assets acquired	30,890
Fixed assets	50
Goodwill	19,108
Purchased customer accounts	7,300
Non-compete agreements	1
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	14
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	31,399
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(480)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	(29)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	509
Servco Pacific, Inc. [Member] | Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	29,106
Other payable	100
Recorded earn-out payable	1,684
Net assets acquired	30,890
Maximum potential earn- out payable	2,500
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [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)
Net assets acquired	29,320
Fixed assets	(97)
Goodwill	22,111
Purchased customer accounts	9,013
Non-compete agreements	106
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	(732)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	23,743
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	5,577
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(5,577)
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	24,980
Other payable	1,126
Recorded earn-out payable	3,214
Net assets acquired	29,320
Maximum potential earn- out payable	7,709
United Development Systems, Inc. [Member]
Business Combination, Separately Recognized Transactions [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
Net assets acquired	22,643
Fixed assets	20
Goodwill	15,111
Purchased customer accounts	7,065
Non-compete agreements	11
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	22,684
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(41)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	41
United Development Systems, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	18,987
Other payable	388
Recorded earn-out payable	3,268
Net assets acquired	22,643
Maximum potential earn- out payable	8,625
Twinbrook Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [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
Net assets acquired	28,216
Fixed assets	85
Goodwill	19,839
Purchased customer accounts	8,557
Non-compete agreements	12
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	28,493
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(277)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	277
Twinbrook Insurance Agency, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash paid	26,251
Other payable	400
Recorded earn-out payable	1,565
Net assets acquired	28,216
Maximum potential earn- out payable	5,073
Maximum [Member] | Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net assets acquired	$ 10,000