Supplemental Information Concerning Insurance Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	$ 464.5	$ 453.3	$ 448.6
Reserves for Unpaid Claims and Claim Adjustment Expenses	12,250.3	11,871.3	11,087.2
Discount if Any Deducted, in Reserves for Unpaid Claims and Claim Adjustment Expenses	0.0	0.0	0.0
Unearned Premiums	2,267.1	2,182.3	2,175.5
Earned Premiums	4,976.2	4,955.0	4,975.8
Net Investment Income	480.9	434.6	433.1
Claims and Claim Adjustment Expense Incurred Related to Current Year	3,849.4	3,918.8	3,285.2
Claims and Claim Adjustment Expense Incurred Related to Prior Year	(329.0)	(298.6)	(368.0)
Amortization of Deferred Policy Acquisition Costs	1,271.4	1,261.7	1,253.2
Paid Claims and Claim Adjustment Expenses	3,257.9	3,078.4	2,600.8
Premiums Written	$ 5,048.4	$ 4,965.9	$ 5,091.8