Identifiable Assets for Business Segments and Discontinued Operations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Discontinued operations	$ 129.8	$ 121.2
Total assets	13,330.1	12,598.6
U.S. Companies
Segment Reporting Information [Line Items]
Identifiable assets	8,799.6	8,471.6
Chaucer
Segment Reporting Information [Line Items]
Identifiable assets	$ 4,400.7	$ 4,005.8