Business Combinations - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) acquisition	Dec. 31, 2016 USD ($) acquisition	Dec. 31, 2015 USD ($) acquisition customer_account	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Maximum Future Contingency payments Acquisitions	$ 88,400,000
Number of acquisitions | acquisition	11	8
Aggregate purchase price of acquisitions	$ 1,500,000	$ 917,497	$ 503,442
Payments to Acquire Businesses, Gross	41,471,000	124,716,000	136,000,000.0
Goodwill assigned	42,172,000	93,376,000	136,381,000
Goodwill currently deductible for income tax purposes	35,300,000	88,900,000	91,100,000
Goodwill currently non-deductible for income tax purposes			8,400,000
Goodwill related to the recorded earn-out payables	6,900,000	4,500,000	36,900,000
Total revenues related to acquisitions	7,800,000	34,200,000	28,200,000
Income before income taxes related to acquisitions	2,400,000	4,300,000	1,500,000
Estimated acquisition earn-out payables	36,175,000	63,821,000	78,387,000	$ 75,283,000
Other Payable	11,708,000	10,664,000	10,028,000
Business Acquisitions Contingent Consideration At Fair Value	6,921,000	4,462,000	36,899,000
Net Assets Acquired	60,100,000	140,334,000	182,927,000
Maximum Potential Earn- Out Payable	27,451,000	16,285,000	73,074,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 25,100,000	$ 31,800,000	$ 25,300,000
Purchased customer accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years	15 years	15 years
Non-compete agreements
Business Acquisition [Line Items]
Weighted average life (years)	5 years	5 years	5 years
National Programs
Business Acquisition [Line Items]
Goodwill assigned	$ 7,200,000	$ 57,900,000	$ 18,000,000.0
Wholesale Brokerage [Member]
Business Acquisition [Line Items]
Goodwill assigned	1,200,000	(1,200)	4,600,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned	700,000	22,400,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 11,100,000	32,000,000.0	$ 53,100,000
Social Security Advocates for the Disabled LLC [Member]
Business Acquisition [Line Items]
Goodwill assigned		22,352,000
Net Assets Acquired		$ 33,989,000
Social Security Advocates for the Disabled LLC [Member] | Services [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition		Feb. 01, 2016
Payments to Acquire Businesses, Gross		$ 32,526,000
Other Payable		0
Business Acquisitions Contingent Consideration At Fair Value		971,000
Net Assets Acquired		33,989,000
Maximum Potential Earn- Out Payable		3,500,000
Morstan General Agency, Inc [Member]
Business Acquisition [Line Items]
Goodwill assigned		51,454,000
Net Assets Acquired		$ 79,341,000
Morstan General Agency, Inc [Member] | Wholesale Brokerage [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition		Jun. 01, 2016
Payments to Acquire Businesses, Gross		$ 66,050,000
Other Payable		10,200,000
Business Acquisitions Contingent Consideration At Fair Value		3,091,000
Net Assets Acquired		79,341,000
Maximum Potential Earn- Out Payable		$ 5,000,000
Asset Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	11	7	13
Book of Business Purchases [Member]
Business Acquisition [Line Items]
Number of acquisitions	1	3	4
Other Acquisitions [Member]
Business Acquisition [Line Items]
Goodwill assigned			$ 21,040,000
Net Assets Acquired			17,605,000
Other Acquisitions [Member] | Various
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 41,471,000	$ 26,140,000	12,926,000
Other Payable	11,708,000	464,000	95,000
Business Acquisitions Contingent Consideration At Fair Value	6,921,000	400,000	4,584,000
Net Assets Acquired	60,100,000	27,004,000	17,605,000
Maximum Potential Earn- Out Payable	$ 27,451,000	$ 7,785,000	$ 8,212,000