Fair Value (Fair Value On Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	$ 98.9	$ 67.5
Debt	997.6
Transfers into Level 3	4.4	3.7
Transfers out of Level 3	(7.7)
Included in earnings	(0.2)	(0.5)
Included in other comprehensive income	(0.7)	0.2
Purchases	8.2	2.9
Sales	(6.3)	(1.4)
Ending balance	96.6	72.4
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	75.5	64.6
Transfers into Level 3	4.3	3.7
Transfers out of Level 3	(7.7)
Included in other comprehensive income	0.5	(0.3)
Purchases	8.2	2.9
Sales	(5.6)	(1.4)
Ending balance	75.2	69.5
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	23.4	2.9
Transfers into Level 3	0.1
Included in earnings	(0.2)	(0.5)
Included in other comprehensive income	(1.2)	0.5
Sales	(0.7)
Ending balance	21.4	2.9
Municipal [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	13.6	16.6
Included in other comprehensive income	0.6	(0.4)
Purchases	3.0
Sales	(0.2)	(0.3)
Ending balance	17.0	15.9
Corporate [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	23.8	28.2
Transfers into Level 3	4.3	3.7
Included in earnings	0.1
Included in other comprehensive income	0.8	0.1
Purchases		2.9
Sales	(0.3)	(0.6)
Ending balance	28.7	34.3
Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	5.2	0.8
Included in other comprehensive income	0.1
Sales	(4.6)	(0.2)
Ending balance	0.7	0.6
Commercial Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	4.7	5.5
Sales	(0.2)	(0.2)
Ending balance	4.5	5.3
Asset-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	28.2	13.5
Transfers out of Level 3	(7.7)
Included in earnings	(0.1)
Included in other comprehensive income	(1.0)
Purchases	5.2
Sales	(0.3)	(0.1)
Ending balance	$ 24.3	$ 13.4