Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Retail [Member]	Jun. 30, 2013 National Programs [Member]	Jun. 30, 2013 Wholesale Brokerage [Member]	Jun. 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	11,314	11,984	173	(843)
Ending balance	$ 1,722,828	$ 888,203	$ 439,353	$ 287,211	$ 108,061	$ 108,061