Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	$ 9,515,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	139,653,000
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	2,094,000
Cash Paid	115,313,000	$ 109,900,000
Business Acquisition Cost of Entity Note Payable	492,000
Other Payable	10,505,000
Recorded Earn-Out Payable	3,828,000
Net Assets Acquired	130,138,000
Maximum Potential Earn- Out Payable	13,722,000
Other current assets	4,277,000
Fixed assets	654,000
Goodwill	85,682,000
Purchased customer accounts	46,751,000
Non-compete agreements	195,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	6,285,000
Deferred Tax Liabilities, Net	3,230,000
Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	22,352,000
Services
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	11,854,000
National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	(1,000)
Wholesale Brokerage
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill	51,477,000
SSAD [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	4,947,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	38,936,000
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	2,094,000
Net Assets Acquired	33,989,000
Other current assets	1,042,000
Fixed assets	307,000
Goodwill	22,352,000
Purchased customer accounts	13,069,000
Non-compete agreements	72,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,717,000
Deferred Tax Liabilities, Net	$ 3,230,000
SSAD [Member] | Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	Feb. 01, 2016
Cash Paid	$ 32,526,000
Business Acquisition Cost of Entity Note Payable	492,000
Other Payable	0
Recorded Earn-Out Payable	971,000
Net Assets Acquired	33,989,000
Maximum Potential Earn- Out Payable	3,000,000
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	3,153,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	19,961,000
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	16,808,000
Other current assets	753,000
Fixed assets	47,000
Goodwill	11,876,000
Purchased customer accounts	7,201,000
Non-compete agreements	84,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	3,153,000
Deferred Tax Liabilities, Net	0
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	16,737,000
Business Acquisition Cost of Entity Note Payable	0
Other Payable	305,000
Recorded Earn-Out Payable	(234,000)
Net Assets Acquired	16,808,000
Maximum Potential Earn- Out Payable	5,722,000
Morstan General Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	1,415,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	80,756,000
Net Assets Acquired	79,341,000
Other current assets	2,482,000
Fixed assets	300,000
Goodwill	51,454,000
Purchased customer accounts	26,481,000
Non-compete agreements	39,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,415,000
Deferred Tax Liabilities, Net	$ 0
Morstan General Agency, Inc. [Member] | Services
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	Jun. 01, 2016
Cash Paid	$ 66,050,000
Business Acquisition Cost of Entity Note Payable	0
Other Payable	10,200,000
Recorded Earn-Out Payable	3,091,000
Net Assets Acquired	79,341,000
Maximum Potential Earn- Out Payable	$ 5,000,000