Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Goodwill [Line Items]
Total goodwill-net	$ 2,145.2	$ 1,472.7	$ 1,155.3
Unites States [Member]
Goodwill [Line Items]
Total goodwill-net	1,469.8	1,177.3
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill-net	597.2	226.0
Other Foreign, Principally Australia and Canada [Member]
Goodwill [Line Items]
Total goodwill-net	78.2	69.4
Brokerage [Member]
Goodwill [Line Items]
Total goodwill-net	2,122.9	1,451.4	1,136.6
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill-net	1,449.6	1,158.1
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill-net	595.1	223.9
Brokerage [Member] | Other Foreign, Principally Australia and Canada [Member]
Goodwill [Line Items]
Total goodwill-net	78.2	69.4
Risk Management [Member]
Goodwill [Line Items]
Total goodwill-net	22.3	21.3	18.7
Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill-net	20.2	19.2
Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill-net	2.1	2.1
Risk Management [Member] | Other Foreign, Principally Australia and Canada [Member]
Goodwill [Line Items]
Total goodwill-net	0	0
Corporate [Member]
Goodwill [Line Items]
Total goodwill-net	0	0
Corporate [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill-net	0	0
Corporate [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill-net	0	0
Corporate [Member] | Other Foreign, Principally Australia and Canada [Member]
Goodwill [Line Items]
Total goodwill-net	$ 0	$ 0