Business Combinations - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Y	Dec. 30, 2011	Dec. 31, 2011 Y	Dec. 31, 2010 Entity Y
Business Acquisition [Line Items]
Aggregate purchase price of acquisitions	$ 667,586,000		$ 214,822,000	$ 186,783,000
Cash payments for acquisitions	483,933,000		167,444,000	158,636,000
Net additional consideration paid as note payable	59,000		1,194,000	759,000
Other Payable	25,439,000
Assumption of liabilities	136,676,000		15,659,000	2,298,000
Recorded earn-out payable	21,479,000		30,525,000	25,090,000
Average annual operating profit earned period, minimum (years)	1		1	1
Average annual operating profit earned period, maximum (years)	3		3	3
Number of acquisitions				33
Total goodwill	390,640,000		128,344,000	116,393,000
Goodwill currently deductible for income tax purposes	52,730,000		84,105,000	91,303,000
Goodwill currently non-deductible for income tax purposes	316,431,000		13,714,000
Goodwill related to the recorded earn-out payables	21,479,000		30,525,000	25,090,000
Total revenues related to acquisitions	129,472,000	40,291,000		30,172,000
Income before income taxes related to acquisitions	898,000	7,223,000		3,255,000
Maximum future contingency payments related to acquisitions	146,858,000
Percentage recognition by acquirer of fair value of acquired assets	100.00%
Estimated acquisition earn-out payables	52,987,000		47,715,000
Other Non-Current Liability
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	42,823,000		44,061,000
Accounts Payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	10,164,000		3,654,000
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
Arrowhead General Insurance Agency Superholding Corporation
Business Acquisition [Line Items]
Other Payable	22,061,000
Purchased customer accounts
Business Acquisition [Line Items]
Weighted average life (years)	15 years		15 years
Non-compete agreements
Business Acquisition [Line Items]
Weighted average life (years)	5 years		5 years
Retail
Business Acquisition [Line Items]
Business acquisitions during period purchase price allocation goodwill expected tax deductible amount	57,856,000		108,420,000	57,423,000
National Programs Divisions
Business Acquisition [Line Items]
Business acquisitions during period purchase price allocation goodwill expected tax deductible amount	289,378,000		11,853,000
Wholesale Brokerage Divisions
Business Acquisition [Line Items]
Business acquisitions during period purchase price allocation goodwill expected tax deductible amount	11,656,000
Services
Business Acquisition [Line Items]
Business acquisitions during period purchase price allocation goodwill expected tax deductible amount	31,750,000		8,071,000	58,970,000
Subsequent to January 1, 2009
Business Acquisition [Line Items]
Maximum future contingency payments related to acquisitions	146,858,000
Future earnings of acquired entities, term basis of potential earn-out obligations, minimum (years)	1
Future earnings of acquired entities, term basis of potential earn-out obligations, maximum (years)	3
Prior to January 1, 2009
Business Acquisition [Line Items]
Cash payments for acquisitions	2,907,000		3,781,000	975,000
Net additional consideration paid as note payable			409,000	3,062,000
Additional net consideration paid allocated to goodwill	$ 2,907,000		$ 4,190,000	$ 4,037,000
Asset Acquisitions
Business Acquisition [Line Items]
Number of acquisitions	19		37
Stock Acquisitions
Business Acquisition [Line Items]
Number of acquisitions	1		1