Fair Value (Fair Value On Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	$ 67.5	$ 62.6
Transfers into Level 3	14.5	16.8
Transfers out of Level 3	(25.1)	(4.7)
Included in earnings	(2.1)	0.4
Included in other comprehensive income	(1.2)	(0.7)
Purchases	25.1	10.7
Sales	(18.5)	(17.6)
Balance at end of year	98.9	67.5
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	64.6	59.8
Transfers into Level 3	14.5	16.8
Transfers out of Level 3	(25.1)	(4.7)
Included in earnings	(0.7)	0.7
Included in other comprehensive income	1.1	(0.1)
Purchases	25.1	9.7
Sales	(18.5)	(17.6)
Balance at end of year	75.5	64.6
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	2.9	2.8
Included in earnings	(1.4)	(0.3)
Included in other comprehensive income	(2.3)	(0.6)
Purchases		1.0
Balance at end of year	23.4	2.9
Municipal [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	16.6	15.5
Included in earnings	(0.1)
Included in other comprehensive income	0.2	(0.5)
Purchases		3.0
Sales	(3.1)	(1.4)
Balance at end of year	13.6	16.6
Corporate [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	28.2	28.9
Transfers into Level 3	14.5	9.9
Transfers out of Level 3	(17.3)	(2.7)
Included in earnings	(0.7)	0.7
Included in other comprehensive income	0.3	0.4
Purchases	11.8	3.3
Sales	(13.1)	(12.3)
Balance at end of year	23.8	28.2
Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	0.8
Transfers out of Level 3	(0.5)
Included in other comprehensive income	(0.1)
Purchases		1.4
Sales	(0.6)	(0.6)
Balance at end of year	5.2	0.8
Commercial Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	5.5	6.2
Transfers out of Level 3	(7.3)
Included in other comprehensive income	(0.1)
Purchases	7.3
Sales	(0.7)	(0.7)
Balance at end of year	4.7	5.5
Asset-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	13.5	9.2
Transfers into Level 3		6.9
Transfers out of Level 3		(2.0)
Included in earnings	0.1
Included in other comprehensive income	0.8
Purchases	6.0	2.0
Sales	(1.0)	(2.6)
Balance at end of year	28.2	13.5
Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	38.7
Chaucer [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	14.5
Chaucer [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	24.2
Chaucer [Member] | Corporate [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	0.1
Chaucer [Member] | Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	5.6
Chaucer [Member] | Asset-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	$ 8.8