Investments - Schedule of Investments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,342.6
Amortized Cost Basis	480.6	462.7
Fixed Maturities at Fair Value	4,856.5	4,860.2
Fair Value	554.1	521.9
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	101.3
Due in One Year or Less - Fair Value	104.5
Due after One Year to Five Years - Amortized Cost	562.1
Due after One Year to Five Years - Fair Value	592.3
Due after Five Years to Ten Years - Amortized Cost	1,273.4
Due after Five Years to Ten Years - Fair Value	1,379.6
Due after Ten Years - Amortized Cost	2,177.2
Due after Ten Years - Fair Value	2,534.3
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	228.6
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	245.8
Other Receivables	202.7	206.1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	514.8	343.9
Less Than 12 Months, Unrealized Losses	(10.8)	(7.6)
12 Months or Longer, Fair Value	74.5	54.6
12 Months or Longer, Unrealized Losses	(4.3)	(3.8)
Total, Fair Value	589.3	398.5
Total, Unrealized Losses	(15.1)	(11.4)
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securites in An Unrealized Loss Position, In a Position of 12 Months or Longer	0	0
Less Than 12 Months, Fair Value	(0.5)	(0.3)
Available For Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses Related to Securities that have Previously Recognized Foreign Currency Impairment	0.1
Other Liabilities	482.5	431.4
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,342.6	4,283.8
Gross Unrealized Gains	524.9	584.5
Gross Unrealized Losses	(11.0)	(8.1)
Fixed Maturities at Fair Value	4,856.5	4,860.2
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	444.4	315.6
Less Than 12 Months, Unrealized Losses	(8.0)	(5.5)
12 Months or Longer, Fair Value	62.9	40.5
12 Months or Longer, Unrealized Losses	(3.0)	(2.6)
Total, Fair Value	507.3	356.1
Total, Unrealized Losses	(11.0)	(8.1)
Other Liabilities	29.8	1.5
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	24.5
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	389.2	384.0
Gross Unrealized Gains	40.9	45.1
Gross Unrealized Losses	(1.0)	(0.2)
Fixed Maturities at Fair Value	429.1	428.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	68.9	40.8
Less Than 12 Months, Unrealized Losses	(1.0)	(0.2)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	68.9	40.8
Total, Unrealized Losses	(1.0)	(0.2)
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	195.3
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	17.1
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	0.5
Other Issuers [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	32.9
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,258.9	1,251.0
Gross Unrealized Gains	143.2	150.5
Gross Unrealized Losses	(0.3)	(0.1)
Fixed Maturities at Fair Value	1,401.8	1,401.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	41.1	6.3
Less Than 12 Months, Unrealized Losses	(0.3)	(0.1)
12 Months or Longer, Fair Value	0.3	0.3
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	41.4	6.6
Total, Unrealized Losses	(0.3)	(0.1)
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	0.6
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fixed Maturities at Fair Value	0.6
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,651.0	2,615.5
Gross Unrealized Gains	337.3	385.4
Gross Unrealized Losses	(9.2)	(7.5)
Fixed Maturities at Fair Value	2,979.1	2,993.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	313.1	268.5
Less Than 12 Months, Unrealized Losses	(6.3)	(5.2)
12 Months or Longer, Fair Value	60.6	38.1
12 Months or Longer, Unrealized Losses	(2.9)	(2.3)
Total, Fair Value	373.7	306.6
Total, Unrealized Losses	(9.2)	(7.5)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	10.8	30.1
Gross Unrealized Gains	2.2	2.5
Gross Unrealized Losses	0	0
Fixed Maturities at Fair Value	13.0	32.6
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0	0
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Fair Value	0.2	0.4
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	0.2	0.4
Total, Unrealized Losses	0	0
Mortgage and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	32.1	3.2
Gross Unrealized Gains	1.3	1.0
Gross Unrealized Losses	(0.5)	(0.3)
Fixed Maturities at Fair Value	32.9	3.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	21.3	0
Less Than 12 Months, Unrealized Losses	(0.4)	0
12 Months or Longer, Fair Value	1.8	1.7
12 Months or Longer, Unrealized Losses	(0.1)	(0.3)
Total, Fair Value	23.1	1.7
Total, Unrealized Losses	(0.5)	(0.3)
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	6.8	2.3
Non-governmental Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	1.3	1.3
Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	0.3	0.3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	480.6	462.7
Gross Unrealized Gains	77.6	62.5
Gross Unrealized Losses	(4.1)	(3.3)
Fair Value	554.1	521.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	70.4	28.3
Less Than 12 Months, Unrealized Losses	(2.8)	(2.1)
12 Months or Longer, Fair Value	11.6	14.1
12 Months or Longer, Unrealized Losses	(1.3)	(1.2)
Total, Fair Value	82.0	42.4
Total, Unrealized Losses	(4.1)	(3.3)
Preferred Stock Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	72.4	75.4
Gross Unrealized Gains	4.8	3.9
Gross Unrealized Losses	(0.1)	(0.1)
Fair Value	77.1	79.2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0	0
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Fair Value	2.4	2.4
12 Months or Longer, Unrealized Losses	(0.1)	(0.1)
Total, Fair Value	2.4	2.4
Total, Unrealized Losses	(0.1)	(0.1)
Preferred Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	14.1	18.4
Gross Unrealized Gains	4.6	3.8
Gross Unrealized Losses	(0.5)	(0.9)
Fair Value	18.2	21.3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	1.3	2.3
Less Than 12 Months, Unrealized Losses	(0.5)	(0.8)
12 Months or Longer, Fair Value	0.8	3.7
12 Months or Longer, Unrealized Losses	0	(0.1)
Total, Fair Value	2.1	6.0
Total, Unrealized Losses	(0.5)	(0.9)
Common Stocks Manufacturing [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	67.4	67.0
Gross Unrealized Gains	29.2	20.9
Gross Unrealized Losses	(0.7)	(0.4)
Fair Value	95.9	87.5
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	3.3	6.3
Less Than 12 Months, Unrealized Losses	(0.6)	(0.4)
12 Months or Longer, Fair Value	0.2	0
12 Months or Longer, Unrealized Losses	(0.1)	0
Total, Fair Value	3.5	6.3
Total, Unrealized Losses	(0.7)	(0.4)
Common Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	48.9	59.1
Gross Unrealized Gains	15.1	8.1
Gross Unrealized Losses	(0.1)	(0.5)
Fair Value	63.9	66.7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	1.6	14.2
Less Than 12 Months, Unrealized Losses	(0.1)	(0.4)
12 Months or Longer, Fair Value	1.9	1.3
12 Months or Longer, Unrealized Losses	0	(0.1)
Total, Fair Value	3.5	15.5
Total, Unrealized Losses	(0.1)	(0.5)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	149.7	119.6
Gross Unrealized Gains	5.4	6.3
Gross Unrealized Losses	0	0
Fair Value	155.1	125.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	5.0
Less Than 12 Months, Unrealized Losses	0
12 Months or Longer, Fair Value	0
12 Months or Longer, Unrealized Losses	0
Total, Fair Value	5.0
Total, Unrealized Losses	0
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	128.1	123.2
Gross Unrealized Gains	18.5	19.5
Gross Unrealized Losses	(2.7)	(1.4)
Fair Value	143.9	141.3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	59.2	5.5
Less Than 12 Months, Unrealized Losses	(1.6)	(0.5)
12 Months or Longer, Fair Value	6.3	6.7
12 Months or Longer, Unrealized Losses	(1.1)	(0.9)
Total, Fair Value	65.5	12.2
Total, Unrealized Losses	(2.7)	(1.4)
External Credit Rating, Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	6.6	3.8
External Credit Rating, Non Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	4.4	4.3
Available For Sale Securities Continuous Unrealized Loss Position Aggregate Losses Related to Securities that have Previously Recognized Foreign Currency Impairment	$ (0.1)	$ (0.1)
Number of Issuers of Available for Sales Securities Continuous Unrealized Loss Position Aggregate Losses Related to Securities that have Previously Recognized Foreign Currency impairment	(1)	1
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	3.00%	3.00%