Supplemental Insurance Information (Detail) - Property and Casualty Insurance - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	$ 448.6	$ 419.4	$ 353.2
Future Policy Benefits, Losses, Claims and Loss Expenses	11,087.2	10,779.2	11,597.2
Unearned Premiums	2,175.5	2,076.1	1,834.2
Other Policy Claims and Benefits Payable	0.0	0.0	0.0
Premium Revenue	4,975.8	4,230.3	4,410.6
Net Investment Income	433.1	427.6	448.9
Benefits, Claims, Losses and Settlement Expenses	2,917.2	2,339.8	2,494.5
Amortization of Deferred Policy Acquisition Costs	1,253.2	1,024.5	1,042.0
Other Operating Expenses	404.1	399.4	379.3
Premiums Written	$ 5,091.8	$ 4,489.2	$ 4,497.5