Business Combinations - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) acquisition	Dec. 31, 2015 USD ($) acquisition customer_account	Dec. 31, 2014 USD ($) acquisition	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Maximum Future Contingency payments Acquisitions	$ 117,200,000
Number of acquisitions | acquisition	8	13
Aggregate purchase price of acquisitions	$ 917,497	$ 503,442	$ 25,941
Payments to Acquire Businesses, Gross	124,716,000	136,000,000	721,851,000
Business Acquisition Cost- Note Payable	492,000
Goodwill assigned	93,376,000	136,381,000	513,925,000
Goodwill currently deductible for income tax purposes	88,900,000	91,100,000	141,900,000
Goodwill currently non-deductible for income tax purposes		8,400,000	338,800,000
Goodwill related to the recorded earn-out payables	4,500,000	36,900,000	33,200,000
Total revenues related to acquisitions	34,200,000	28,200,000	112,200,000
Income before income taxes related to acquisitions	4,300,000	1,500,000	(1,300,000)
Estimated acquisition earn-out payables	63,821,000	78,387,000	75,283,000	$ 43,058,000
Other Payable	10,664,000	10,028,000	1,904,000
Business Acquisitions Contingent Consideration At Fair Value	4,462,000	36,899,000	33,229,000
Net Assets Acquired	140,334,000	182,927,000	756,984,000
Maximum Potential Earn- Out Payable	16,285,000	73,074,000	49,462,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 31,800,000	$ 25,300,000	$ 26,000,000
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	3 years 5 months
National Programs
Business Acquisition [Line Items]
Goodwill assigned	$ 57,900,000	$ 18,000,000	$ 420,000,000
Wholesale Brokerage [Member]
Business Acquisition [Line Items]
Goodwill assigned	0	4,600,000	7,700,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned	22,400,000		(200,000)
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	32,000,000	$ 53,100,000	$ 49,300,000
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
Business Acquisition Cost- Note Payable	492,000
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
Business Acquisition Cost- Note Payable	0
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		13	9
Book of Business Purchases [Member]
Business Acquisition [Line Items]
Number of acquisitions	3	4	5
Stock Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions | acquisition	1	0	1
Other Acquisitions [Member]
Business Acquisition [Line Items]
Goodwill assigned		$ 21,040,000	$ 10,417,000
Net Assets Acquired		17,605,000	$ 17,184,000
Other Acquisitions [Member] | Various
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 26,140,000	12,926,000
Business Acquisition Cost- Note Payable	0
Other Payable	464,000	95,000
Business Acquisitions Contingent Consideration At Fair Value	400,000	4,584,000
Net Assets Acquired	27,004,000	17,605,000
Maximum Potential Earn- Out Payable	$ 7,785,000	$ 8,212,000