Business Combinations - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Jan. 09, 2012
Business Acquisition [Line Items]
Aggregate purchase price of acquisitions	$ 484,319,000	$ 620,144,000	$ 484,319,000	$ 620,144,000
Cash payment for acquisition			379,694,000	443,475,000
Other payables for acquisition			85,000	25,776,000
Assumption of liabilities			101,755,000	135,779,000
Recorded earn-out payables			2,785,000
Average annual operating profit earned period, minimum (years)			1 year	1 year
Average annual operating profit earned period, maximum (years)			3 years	3 years
Aggregate purchase price of acquisitions			(1,084,000)
Cash payments for acquisitions			18,000
Net consideration paid as other note payable			(454,000)
Assumption of liabilities			73,000
Recorded earn-out payable			(721,000)
Total goodwill	278,553,000	361,118,000	278,553,000	361,118,000
Goodwill currently deductible for income tax purposes	278,553,000	28,326,000	278,553,000	28,326,000
Goodwill currently non-deductible for income tax purposes	24,863,000	317,737,000	24,863,000	317,737,000
Goodwill related to the recorded earn-out payables	2,064,000	15,055,000	2,064,000	15,055,000
Total revenues related to acquisitions	29,095,000	37,754,000	29,723,000	96,020,000
Income (loss) before income taxes related to acquisitions	(3,453,000)	4,731,000	(3,311,000)	4,279,000
Net additional consideration paid as note payable		59,000		59,000
Recorded earn-out payable			2,064,000	15,055,000
Maximum future contingency payments related to acquisitions	135,122,000		135,122,000
Percentage recognition by acquirer of fair value of acquired assets			100.00%
Estimated acquisition earn-out payables	46,690,000	58,248,000	46,690,000	58,248,000	48,918,000	52,987,000	57,997,000	47,715,000
Arrowhead General Insurance Agency Superholding Corporation [Member]
Business Acquisition [Line Items]
Other payables for acquisition				22,594,000
Total goodwill	(454,000)	322,779,000	(454,000)	322,779,000
Other Non-Current Liability [Member]
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	35,781,000	40,696,000	35,781,000	40,696,000
Accounts Payable [Member]
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	10,909,000	17,552,000	10,909,000	17,552,000
Subsequent to January 1, 2009 [Member]
Business Acquisition [Line Items]
Maximum future contingency payments related to acquisitions	135,122,000		135,122,000
Future earnings of acquired entities, term basis of potential earn-out obligations, minimum (years)			1 year
Future earnings of acquired entities, term basis of potential earn-out obligations, maximum (years)			3 years
Retail [Member]
Business Acquisition [Line Items]
Goodwill assigned	252,753,000	26,753,000	252,753,000	26,753,000
National Programs Divisions [Member]
Business Acquisition [Line Items]
Goodwill assigned	26,612,000	253,766,000	26,612,000	253,766,000
Wholesale Brokerage Divisions [Member]
Business Acquisition [Line Items]
Goodwill assigned	(812,000)	11,586,000	(812,000)	11,586,000
Services [Member]
Business Acquisition [Line Items]
Goodwill assigned		69,013,000		69,013,000
Prior to January 1, 2009 [Member]
Business Acquisition [Line Items]
Cash payment for acquisition			873,000	2,907,000
Net additional consideration paid as note payable	873,000		873,000
Additional net consideration paid allocated to goodwill			$ 873,000	$ 2,907,000
Purchased customer accounts [Member]
Business Acquisition [Line Items]
Weighted average life (years)			15 years	15 years
Non-compete agreements [Member]
Business Acquisition [Line Items]
Weighted average life (years)			5 years	5 years
Asset Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions			3	11
Stock Acquisitions [Member]
Business Acquisition [Line Items]
Number of acquisitions			1	1