Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill [Line Items]
Total goodwill - net	$ 3,449.6	$ 2,145.2	$ 1,472.7
Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	1,672.8	1,469.8
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	820.6	584.9
Canada [Member]
Goodwill [Line Items]
Total goodwill - net	318.5	26.8
Australia [Member]
Goodwill [Line Items]
Total goodwill - net	336.8	37.1
Other Foreign, Principally New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	300.9	26.6
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	3,427.5	2,122.9	1,451.4
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	1,652.6	1,449.6
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	818.7	582.8
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	318.5	26.8
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	336.8	37.1
Brokerage [Member] | Other Foreign, Principally New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	300.9	26.6
Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	22.1	22.3	21.3
Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	20.2	20.2
Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	1.9	2.1
Risk Management [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Risk Management [Member] | Other Foreign, Principally New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	0	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] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Corporate [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	0	0
Corporate [Member] | Other Foreign, Principally New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	$ 0	$ 0