Changes in Carrying Value of Goodwill by Operating Segment (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Goodwill [Line Items]
Beginning balance	$ 1,323,469	$ 1,194,827
Goodwill of acquired businesses	393,547	132,534
Goodwill disposed of relating to sales of businesses	(5,502)	(3,892)
Ending balance	1,711,514	1,323,469
Retail
Goodwill [Line Items]
Beginning balance	823,573	716,626
Goodwill of acquired businesses	58,148	112,610
Goodwill transferred		(1,771)
Goodwill disposed of relating to sales of businesses	(5,502)	(3,892)
Ending balance	876,219	823,573
National Programs
Goodwill [Line Items]
Beginning balance	149,802	140,238
Goodwill of acquired businesses	289,378	9,564
Ending balance	439,180	149,802
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	273,783	269,723
Goodwill of acquired businesses	14,271	2,289
Goodwill transferred		1,771
Ending balance	288,054	273,783
Services
Goodwill [Line Items]
Beginning balance	76,311	68,240
Goodwill of acquired businesses	31,750	8,071
Ending balance	$ 108,061	$ 76,311