Business Combinations - Acquisitions Accounted for Business Combinations (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
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	(4,345)
Cash Paid	98,299
Other Payable	608
Recorded Earn-Out Payable	18,155
Net Assets Acquired	117,062
Maximum Potential Earn- Out Payable	27,521
Fixed assets	112
Goodwill	80,432
Purchased customer accounts	35,782
Non-compete agreements	138
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	316
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	115,653
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	$ 1,438
Document Period End Date	Mar. 31, 2019
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	$ (29)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(1,409)
Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	19,200
Services
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	55,100
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	588
Net Assets Acquired	22,833
Fixed assets	40
Goodwill	16,031
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	23,200
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(367)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	367
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	194
Net Assets Acquired	26,376
Fixed assets	114
Goodwill	18,146
Purchased customer accounts	11,360
Non-compete agreements	11
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	29,825
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(3,449)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	3,449
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	0
Net Assets Acquired	15,710
Fixed assets	29
Goodwill	11,336
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	30,790
Fixed assets	50
Goodwill	19,008
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	31,299
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	0
Recorded Earn-Out Payable	1,684
Net Assets Acquired	30,790
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	(6,835)
Net Assets Acquired	21,353
Fixed assets	(121)
Goodwill	15,911
Purchased customer accounts	6,298
Non-compete agreements	64
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	302
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	15,619
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	5,734
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(5,734)
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	18,654
Other Payable	463
Recorded Earn-Out Payable	2,236
Net Assets Acquired	21,353
Maximum Potential Earn- Out Payable	$ 5,475