Business Combinations - Acquisitions Accounted for Business Combinations (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	$ 11,303
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	13,407
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(22,204)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	343,150
Cash Paid	265,890
Other Payable	3,339
Recorded Earn-Out Payable	51,717
Net Assets Acquired	320,946
Maximum Potential Earn- Out Payable	97,895
Fixed assets	832
Goodwill	214,559
Purchased customer accounts	101,781
Non-compete agreements	506
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	762
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	$ (21,639)
Document Period End Date	Sep. 30, 2018
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	$ (565)
Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	17,300
Services
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	173,000
National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	18,700
Wholesale Brokerage
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	5,500
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	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	22,984
Net Assets Acquired	22,984
Fixed assets	11
Goodwill	17,944
Purchased customer accounts	5,008
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	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	2,384
Net Assets Acquired	22,984
Maximum Potential Earn- Out Payable	3,600
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	0
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 Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(1,414)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
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 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	1,500
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	47,575
Net Assets Acquired	47,575
Fixed assets	67
Goodwill	36,236
Purchased customer accounts	9,751
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Automotive Development Group, LLC [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	29,471
Other Payable	559
Recorded Earn-Out Payable	17,545
Net Assets Acquired	47,575
Maximum Potential Earn- Out Payable	20,000
Servco Pacific, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	8,189
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	8,075
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(11,307)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	88,774
Net Assets Acquired	77,467
Fixed assets	178
Goodwill	53,901
Purchased customer accounts	16,902
Non-compete agreements	1
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	1,528
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(11,307)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
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
Tower Hill Prime Insurance Company [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
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	22,928
Net Assets Acquired	22,928
Fixed assets	0
Goodwill	0
Purchased customer accounts	22,928
Non-compete agreements	0
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Tower Hill Prime Insurance Company [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	20,300
Other Payable	0
Recorded Earn-Out Payable	2,628
Net Assets Acquired	22,928
Maximum Potential Earn- Out Payable	7,700
Health Special Risk, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	3,114
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	818
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(6,272)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	28,395
Net Assets Acquired	22,123
Fixed assets	124
Goodwill	18,737
Purchased customer accounts	5,516
Non-compete agreements	65
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	21
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(5,930)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	(342)
Health Special Risk, Inc. [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	20,132
Other Payable	0
Recorded Earn-Out Payable	1,991
Net Assets Acquired	22,123
Maximum Potential Earn- Out Payable	9,000
Professional Disability Associates, 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	1,762
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(1,496)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	26,339
Net Assets Acquired	24,843
Fixed assets	309
Goodwill	17,273
Purchased customer accounts	7,700
Non-compete agreements	82
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	(787)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(1,273)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	(223)
Professional Disability Associates, LLC [Member] | Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	15,025
Other Payable	0
Recorded Earn-Out Payable	9,818
Net Assets Acquired	24,843
Maximum Potential Earn- Out Payable	17,975
Finance & Insurance Resources, 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	999
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	54,471
Net Assets Acquired	54,471
Fixed assets	34
Goodwill	36,806
Purchased customer accounts	16,611
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Finance & Insurance Resources, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	44,940
Other Payable	410
Recorded Earn-Out Payable	9,121
Net Assets Acquired	54,471
Maximum Potential Earn- Out Payable	19,500
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	253
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	(1,715)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	33,523
Net Assets Acquired	31,808
Fixed assets	99
Goodwill	20,245
Purchased customer accounts	12,653
Non-compete agreements	273
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	(1,715)
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	25,894
Other Payable	680
Recorded Earn-Out Payable	5,234
Net Assets Acquired	31,808
Maximum Potential Earn- Out Payable	$ 10,200