Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Retail [Member]	Sep. 30, 2013 National Programs [Member]	Sep. 30, 2013 Wholesale Brokerage [Member]	Sep. 30, 2013 Services [Member]	Dec. 31, 2012 Services [Member]
Goodwill [Line Items]
Beginning balance	$ 1,711,514	$ 876,219	$ 439,180	$ 288,054	$ 108,061	$ 108,061
Goodwill of acquired businesses	279,426	252,753	27,485	(812)
Ending balance	$ 1,990,940	$ 1,128,972	$ 466,665	$ 287,242	$ 108,061	$ 108,061