Business Combinations - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2015 USD ($) acquisition customer_account	Jun. 30, 2014 USD ($) acquisition	Apr. 01, 2015 USD ($)	Jan. 01, 2015 USD ($)	Apr. 01, 2014 USD ($)	Jan. 01, 2014 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | acquisition		7	6
Number of customer accounts acquired | customer_account		3
Aggregate purchase price of acquisitions		$ 503,442
Payments to Acquire Businesses, Gross		105,056,000	$ 720,100,000
Goodwill assigned	$ 101,509,000	101,509,000
Goodwill currently deductible for income tax purposes	65,974,000	65,974,000
Goodwill currently non-deductible for income tax purposes	8,401,000	8,401,000
Goodwill related to the recorded earn-out payables	27,134,000	27,134,000
Total revenues related to acquisitions	5,696,000	7,422,000
Income before income taxes related to acquisitions	552,000	846,000
Estimated acquisition earn-out payables	90,113,000	90,113,000	$ 52,696,000	$ 77,709,000	$ 75,283,000	$ 48,806,000	$ 43,058,000
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	21,200,000	$ 21,200,000
Purchased customer accounts
Business Acquisition [Line Items]
Weighted average life (years)		15 years
Non-compete agreements
Business Acquisition [Line Items]
Weighted average life (years)		5 years
Retail
Business Acquisition [Line Items]
Goodwill assigned	80,533,000	$ 80,533,000
National Programs
Business Acquisition [Line Items]
Goodwill assigned	18,009,000	18,009,000
Wholesale Brokerage
Business Acquisition [Line Items]
Goodwill assigned	2,967,000	2,967,000
Other non-current liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	$ 68,900,000	$ 68,900,000