Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Total goodwill - net	$ 3,767.8	$ 3,662.9	$ 3,449.6
Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	2,138.5	1,970.4
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	666.5	782.8
Canada [Member]
Goodwill [Line Items]
Total goodwill - net	292.2	282.6
Australia [Member]
Goodwill [Line Items]
Total goodwill - net	382.7	380.1
New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	205.3	204.5
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	82.6	42.5
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	3,736.9	3,635.6	3,427.5
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	2,115.0	1,946.9
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	662.2	779.3
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	292.2	282.6
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	382.7	380.1
Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	205.0	204.2
Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	79.8	42.5
Risk Management Services Management Segment [Member]
Goodwill [Line Items]
Total goodwill - net	28.1	27.3	$ 22.1
Risk Management Services Management Segment [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	23.5	23.5
Risk Management Services Management Segment [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	4.3	3.5
Risk Management Services Management Segment [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	0.3	$ 0.3
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	2.8
Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	$ 2.8