Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Goodwill [Line Items]
Total goodwill - net	$ 1,472.7	$ 1,155.3	$ 883.7
United States [Member]
Goodwill [Line Items]
Total goodwill - net	1,177.3	969.5
Foreign, principally Australia, Canada and the U.K. [Member]
Goodwill [Line Items]
Total goodwill - net	295.4	185.8
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	1,451.4	1,136.6	866.6
Brokerage [Member] | United States [Member]
Goodwill [Line Items]
Total goodwill - net	1,158.1	951.0
Brokerage [Member] | Foreign, principally Australia, Canada and the U.K. [Member]
Goodwill [Line Items]
Total goodwill - net	293.3	185.6
Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	21.3	18.7	17.1
Risk Management [Member] | United States [Member]
Goodwill [Line Items]
Total goodwill - net	19.2	18.5
Risk Management [Member] | Foreign, principally Australia, Canada and the U.K. [Member]
Goodwill [Line Items]
Total goodwill - net	2.1	0.2
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Corporate [Member] | United States [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Corporate [Member] | Foreign, principally Australia, Canada and the U.K. [Member]
Goodwill [Line Items]
Total goodwill - net	$ 0	$ 0