Supplemental Insurance Information (Detail) (Property and Casualty Insurance, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property and Casualty Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	$ 353.2	$ 334.7	$ 303.5
Future Policy Benefits, Losses, Claims and Loss Expenses	11,597.2	11,952.5	12,239.8
Unearned Premiums	1,834.2	1,765.6	1,705.3
Other Policy Claims and Benefits Payable	0	0	0
Premium Revenue	4,410.6	4,239.2	3,733.0
Net Investment Income	448.9	415.2	317.5
Benefits, Claims, Losses and Settlement Expenses	2,494.5	2,479.3	2,630.3
Amortization of Deferred Policy Acquisition Costs	1,042.0	973.5	578.1
Other Operating Expenses	379.3	365.7	304.3
Premiums Written	$ 4,497.5	$ 4,287.4	$ 3,723.9