Business Combinations - Acquisitions Accounted for Business Combinations (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 13,959
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	188,408
Cash Paid	149,992
Other Payable	2,386
Recorded Earn-Out Payable	22,071
Net Assets Acquired	174,449
Maximum Potential Earn- Out Payable	38,048
Fixed assets	290
Goodwill	129,775
Purchased customer accounts	40,713
Non-compete agreements	170
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	$ 13,959
Document Period End Date	Jun. 30, 2018
Services
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	$ 124,200
Wholesale Brokerage
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	5,600
Opus Advisory Group, LLC [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	23,022
Net Assets Acquired	23,022
Fixed assets	12
Goodwill	17,938
Purchased customer accounts	5,051
Non-compete agreements	21
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Opus Advisory Group, LLC [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	20,400
Other Payable	200
Recorded Earn-Out Payable	2,422
Net Assets Acquired	23,022
Maximum Potential Earn- Out Payable	3,600
Kerxton Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	1,414
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	18,161
Net Assets Acquired	16,747
Fixed assets	10
Goodwill	13,417
Purchased customer accounts	4,712
Non-compete agreements	22
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,414
Kerxton Insurance Agency, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	13,177
Other Payable	1,490
Recorded Earn-Out Payable	2,080
Net Assets Acquired	16,747
Maximum Potential Earn- Out Payable	2,920
Automotive Development Group, LLC [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	44,660
Net Assets Acquired	44,660
Fixed assets	67
Goodwill	35,436
Purchased customer accounts	9,136
Non-compete agreements	21
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Automotive Development Group, LLC [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	29,471
Other Payable	559
Recorded Earn-Out Payable	14,630
Net Assets Acquired	44,660
Maximum Potential Earn- Out Payable	20,000
Servco Pacific, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	11,041
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	88,508
Net Assets Acquired	77,467
Fixed assets	178
Goodwill	53,967
Purchased customer accounts	16,902
Non-compete agreements	1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	11,041
Servco Pacific, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	76,551
Other Payable	0
Recorded Earn-Out Payable	916
Net Assets Acquired	77,467
Maximum Potential Earn- Out Payable	7,000
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	1,504
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	14,057
Net Assets Acquired	12,553
Fixed assets	23
Goodwill	9,017
Purchased customer accounts	4,912
Non-compete agreements	105
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,504
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	10,393
Other Payable	137
Recorded Earn-Out Payable	2,023
Net Assets Acquired	12,553
Maximum Potential Earn- Out Payable	$ 4,528