Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Beginning balance	$ 72.4	$ 78.7	$ 67.5	$ 62.6
Transfers into Level 3			3.7	13.5
Transfers out of Level 3	(6.5)	(2.0)	(6.5)	(2.0)
Included in earnings			(0.5)	(0.2)
Included in other comprehensive income	1.0	1.3	1.2	1.7
Purchases	8.9		11.8	3.7
Sales	(0.9)	(6.4)	(2.3)	(7.7)
Ending balance	74.9	71.6	74.9	71.6
Corporate [Member] | Fixed Maturities [Member]
Beginning balance	34.3	35.8	28.2	28.9
Transfers into Level 3			3.7	6.6
Transfers out of Level 3	(6.5)		(6.5)
Included in earnings				0.1
Included in other comprehensive income	0.2	0.7	0.3	1.0
Purchases	8.9		11.8	0.3
Sales	(0.1)	(5.3)	(0.7)	(5.7)
Ending balance	36.8	31.2	36.8	31.2
Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Beginning balance	0.6	1.4	0.8
Transfers into Level 3
Transfers out of Level 3
Included in earnings
Included in other comprehensive income
Purchases				1.4
Sales	(0.1)	(0.3)	(0.3)	(0.3)
Ending balance	0.5	1.1	0.5	1.1
Commercial Mortgage-Backed [Member] | Fixed Maturities [Member]
Beginning balance	5.3	5.9	5.5	6.2
Transfers into Level 3
Transfers out of Level 3
Included in earnings
Included in other comprehensive income		0.1
Purchases
Sales	(0.2)	(0.1)	(0.4)	(0.3)
Ending balance	5.1	5.9	5.1	5.9
Municipal [Member] | Fixed Maturities [Member]
Beginning balance	15.9	15.2	16.6	15.5
Transfers into Level 3
Transfers out of Level 3
Included in earnings
Included in other comprehensive income	0.5	0.1	0.1	0.2
Purchases
Sales	(0.3)	(0.4)	(0.6)	(0.8)
Ending balance	16.1	14.9	16.1	14.9
Asset-Backed [Member] | Fixed Maturities [Member]
Beginning balance	13.4	17.6	13.5	9.2
Transfers into Level 3				6.9
Transfers out of Level 3		(2.0)		(2.0)
Included in earnings
Included in other comprehensive income	0.3	0.4	0.3	0.2
Purchases				2.0
Sales	(0.2)	(0.3)	(0.3)	(0.6)
Ending balance	13.5	15.7	13.5	15.7
Fixed Maturities [Member]
Beginning balance	69.5	75.9	64.6	59.8
Transfers into Level 3			3.7	13.5
Transfers out of Level 3	(6.5)	(2.0)	(6.5)	(2.0)
Included in earnings				0.1
Included in other comprehensive income	1.0	1.3	0.7	1.4
Purchases	8.9		11.8	3.7
Sales	(0.9)	(6.4)	(2.3)	(7.7)
Ending balance	72.0	68.8	72.0	68.8
Equity Securities [Member]
Beginning balance	2.9	2.8	2.9	2.8
Transfers into Level 3
Transfers out of Level 3
Included in earnings			(0.5)	(0.3)
Included in other comprehensive income			0.5	0.3
Purchases
Sales
Ending balance	$ 2.9	$ 2.8	$ 2.9	$ 2.8