Investments - Schedule of Investments (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 4,129.5
Fair Value	4,725.6	4,773.4
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	49.8
Due in One Year or Less - Fair Value	49.9
Due after One Year to Five Years - Amortized Cost	549.1
Due after One Year to Five Years - Fair Value	581.8
Due after Five Years to Ten Years - Amortized Cost	1,045.6
Due after Five Years to Ten Years - Fair Value	1,160.7
Due after Ten Years - Amortized Cost	2,271.1
Due after Ten Years - Fair Value	2,695.9
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	213.9
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	237.3
Other Receivables	207.6	218.7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	228.3	259.5
Less Than 12 Months, Unrealized Losses	(4.9)	(16.7)
12 Months or Longer, Fair Value	64.1	94.6
12 Months or Longer, Unrealized Losses	(5.7)	(6.6)
Total, Fair Value	292.4	354.1
Total, Unrealized Losses	(10.6)	(23.3)
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
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	0.2
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securites in An Unrealized Loss Position	0
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
Other Liabilities	440.1	403.3
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,129.5	4,266.1
Gross Unrealized Gains	603.5	518.4
Gross Unrealized Losses	(7.4)	(11.1)
Fair Value	4,725.6	4,773.4
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	5.1	0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	212.4	173.5
Less Than 12 Months, Unrealized Losses	(3.6)	(5.2)
12 Months or Longer, Fair Value	44.2	89.5
12 Months or Longer, Unrealized Losses	(3.8)	(5.9)
Total, Fair Value	256.6	263.0
Total, Unrealized Losses	(7.4)	(11.1)
Other Liabilities	23.3	0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	406.3	439.4
Gross Unrealized Gains	50.6	52.3
Gross Unrealized Losses	(0.3)	0
Fair Value	456.6	491.7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	40.0	1.3
Less Than 12 Months, Unrealized Losses	(0.3)	0
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	40.0	1.3
Total, Unrealized Losses	(0.3)	0
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	209.9
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	22.0
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	0.7
Other Issuers [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	4.7
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,280.6	1,705.0
Gross Unrealized Gains	155.4	148.4
Gross Unrealized Losses	0	(0.8)
Fair Value	1,436.0	1,852.6
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0	2.0
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Fair Value	0.7	12.0
12 Months or Longer, Unrealized Losses	0	(0.8)
Total, Fair Value	0.7	14.0
Total, Unrealized Losses	0	(0.8)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	2,399.8	2,040.1
Gross Unrealized Gains	393.2	311.6
Gross Unrealized Losses	(6.4)	(9.4)
Fair Value	2,786.6	2,342.3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	172.3	169.6
Less Than 12 Months, Unrealized Losses	(3.3)	(5.1)
12 Months or Longer, Fair Value	40.3	74.7
12 Months or Longer, Unrealized Losses	(3.1)	(4.3)
Total, Fair Value	212.6	244.3
Total, Unrealized Losses	(6.4)	(9.4)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	38.6	76.7
Gross Unrealized Gains	3.3	5.1
Gross Unrealized Losses	(0.2)	(0.1)
Fair Value	41.7	81.7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0	0.6
Less Than 12 Months, Unrealized Losses	0	(0.1)
12 Months or Longer, Fair Value	0.7	0.1
12 Months or Longer, Unrealized Losses	(0.2)	0
Total, Fair Value	0.7	0.7
Total, Unrealized Losses	(0.2)	(0.1)
Mortgage and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4.2	4.9
Gross Unrealized Gains	1.0	1.0
Gross Unrealized Losses	(0.5)	(0.8)
Fair Value	4.7	5.1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0.1	0
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Fair Value	2.5	2.7
12 Months or Longer, Unrealized Losses	(0.5)	(0.8)
Total, Fair Value	2.6	2.7
Total, Unrealized Losses	(0.5)	(0.8)
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	2.9	2.9
Non-governmental Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1.5	1.7
Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0.3	0.4
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value		0.1
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	481.4	367.3
Gross Unrealized Gains	67.5	42.2
Gross Unrealized Losses	(3.2)	(12.2)
Fair Value	545.7	397.3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	15.9	86.0
Less Than 12 Months, Unrealized Losses	(1.3)	(11.5)
12 Months or Longer, Fair Value	19.9	5.1
12 Months or Longer, Unrealized Losses	(1.9)	(0.7)
Total, Fair Value	35.8	91.1
Total, Unrealized Losses	(3.2)	(12.2)
Preferred Stock Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	80.4	94.4
Gross Unrealized Gains	4.5	1.0
Gross Unrealized Losses	(0.2)	(8.7)
Fair Value	84.7	86.7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0	54.9
Less Than 12 Months, Unrealized Losses	0	(8.1)
12 Months or Longer, Fair Value	2.3	2.2
12 Months or Longer, Unrealized Losses	(0.2)	(0.6)
Total, Fair Value	2.3	57.1
Total, Unrealized Losses	(0.2)	(8.7)
Preferred Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	18.4	18.0
Gross Unrealized Gains	3.9	2.6
Gross Unrealized Losses	(0.1)	(0.1)
Fair Value	22.2	20.5
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0.9	1.8
Less Than 12 Months, Unrealized Losses	0	0
12 Months or Longer, Fair Value	3.7	2.8
12 Months or Longer, Unrealized Losses	(0.1)	(0.1)
Total, Fair Value	4.6	4.6
Total, Unrealized Losses	(0.1)	(0.1)
Common Stocks Manufacturing [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	66.5	64.6
Gross Unrealized Gains	23.9	18.9
Gross Unrealized Losses	(0.2)	(0.1)
Fair Value	90.2	83.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	2.3	1.5
Less Than 12 Months, Unrealized Losses	(0.2)	(0.1)
12 Months or Longer, Fair Value	0	0.1
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	2.3	1.6
Total, Unrealized Losses	(0.2)	(0.1)
Common Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	65.0	41.4
Gross Unrealized Gains	13.0	7.4
Gross Unrealized Losses	(0.9)	(1.8)
Fair Value	77.1	47.0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	4.7	10.7
Less Than 12 Months, Unrealized Losses	(0.3)	(1.8)
12 Months or Longer, Fair Value	2.8	0
12 Months or Longer, Unrealized Losses	(0.6)	0
Total, Fair Value	7.5	10.7
Total, Unrealized Losses	(0.9)	(1.8)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	144.6	66.0
Gross Unrealized Gains	7.2	0.6
Gross Unrealized Losses	0	0
Fair Value	151.8	66.6
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	106.5	82.9
Gross Unrealized Gains	15.0	11.7
Gross Unrealized Losses	(1.8)	(1.5)
Fair Value	119.7	93.1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	8.0	17.1
Less Than 12 Months, Unrealized Losses	(0.8)	(1.5)
12 Months or Longer, Fair Value	11.1	0
12 Months or Longer, Unrealized Losses	(1.0)	0
Total, Fair Value	19.1	17.1
Total, Unrealized Losses	(1.8)	(1.5)
External Credit Rating, Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	2.3	(5.7)
External Credit Rating, Non Investment Grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available For Sale Securities Continuous Unrealized Loss Position Aggregate Losses Related to Securities that have Previously Recognized Foreign Currency Impairment	(0.1)	(0.2)
External Credit Rating, Non Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Total, Unrealized Losses	$ 5.1	$ (5.4)
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	4.00%	4.00%