Investments - Schedule of Investments (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013 issuer	Dec. 31, 2012 issuer
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,391.4
Fixed Maturities at Fair Value	4,645.0	4,860.2
Available for sale Securities, Equity Securities, Cost	501.7	462.7
Available for sale Securities, Equity Securities, Fair Value	567.4	521.9
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	104.6
Due in One Year or Less - Fair Value	108.7
Due after One Year to Five Years - Amortized Cost	674.8
Due after One Year to Five Years - Fair Value	713.7
Due after Five Years to Ten Years - Amortized Cost	1,320.4
Due after Five Years to Ten Years - Fair Value	1,352.3
Due after Ten Years - Amortized Cost	2,075.5
Due after Ten Years - Fair Value	2,243.3
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	216.1
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	227.0
Other Receivables	203.9	206.1
Other Liabilities	402.1	431.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	1,244.8	343.9
Less Than 12 Months, Unrealized Losses	(71.1)	(7.6)
12 Months or Longer, Fair Value	79.0	54.6
12 Months or Longer, Unrealized Losses	(6.4)	(3.8)
Total, Fair Value	1,323.8	398.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	77.5	11.4
Less Than 12 Months, Fair Value	(0.5)	(0.3)
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	(0.5)	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
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,391.4	4,283.8
Available-for-sale Debt Securities Gross Unrealized Gain	322.4	584.5
Available-for-sale Debt Securities, Gross Unrealized Loss	68.8	8.1
Fixed Maturities at Fair Value	4,645.0	4,860.2
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0	0
Other Liabilities	0	1.5
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	1,043.7	315.6
Less Than 12 Months, Unrealized Losses	(63.5)	(5.5)
12 Months or Longer, Fair Value	69.5	40.5
12 Months or Longer, Unrealized Losses	(5.3)	(2.6)
Total, Fair Value	1,113.2	356.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	68.8	8.1
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	41.3
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	357.2	384.0
Available-for-sale Debt Securities Gross Unrealized Gain	28.4	45.1
Available-for-sale Debt Securities, Gross Unrealized Loss	9.3	0.2
Fixed Maturities at Fair Value	376.3	428.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	89.4	40.8
Less Than 12 Months, Unrealized Losses	(8.7)	(0.2)
12 Months or Longer, Fair Value	4.6	0
12 Months or Longer, Unrealized Losses	(0.6)	0
Total, Fair Value	94.0	40.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	9.3	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	165.1
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	12.1
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	0.4
Other Non-Governmental Issuers [Member]
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	49.4
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,334.9	1,251.0
Available-for-sale Debt Securities Gross Unrealized Gain	66.9	150.5
Available-for-sale Debt Securities, Gross Unrealized Loss	17.2	0.1
Fixed Maturities at Fair Value	1,384.6	1,401.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	217.0	6.3
Less Than 12 Months, Unrealized Losses	(17.2)	(0.1)
12 Months or Longer, Fair Value	0.3	0.3
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	217.3	6.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	17.2	0.1
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,643.6	2,615.5
Available-for-sale Debt Securities Gross Unrealized Gain	223.6	385.4
Available-for-sale Debt Securities, Gross Unrealized Loss	41.7	7.5
Fixed Maturities at Fair Value	2,825.5	2,993.4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	704.4	268.5
Less Than 12 Months, Unrealized Losses	(37.0)	(5.2)
12 Months or Longer, Fair Value	63.0	38.1
12 Months or Longer, Unrealized Losses	(4.7)	(2.3)
Total, Fair Value	767.4	306.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	41.7	7.5
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	7.1	30.1
Available-for-sale Debt Securities Gross Unrealized Gain	2.1	2.5
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Fixed Maturities at Fair Value	9.2	32.6
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value		0
Less Than 12 Months, Unrealized Losses		0
12 Months or Longer, Fair Value		0.4
12 Months or Longer, Unrealized Losses		0
Total, Fair Value		0.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments		0
Mortgage and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	48.6	3.2
Available-for-sale Debt Securities Gross Unrealized Gain	1.4	1.0
Available-for-sale Debt Securities, Gross Unrealized Loss	0.6	0.3
Fixed Maturities at Fair Value	49.4	3.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	32.9	0
Less Than 12 Months, Unrealized Losses	(0.6)	0
12 Months or Longer, Fair Value	1.6	1.7
12 Months or Longer, Unrealized Losses	0	(0.3)
Total, Fair Value	34.5	1.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0.6	0.3
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	6.7	2.3
Non-governmental Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities at Fair Value	1.1	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]
Available for sale Securities, Equity Securities, Cost	501.7	462.7
Available-for-sale Equity Securities, Gross Unrealized Gain	74.4	62.5
Available-for-sale Equity Securities, Gross Unrealized Loss	8.7	3.3
Available for sale Securities, Equity Securities, Fair Value	567.4	521.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	201.1	28.3
Less Than 12 Months, Unrealized Losses	(7.6)	(2.1)
12 Months or Longer, Fair Value	9.5	14.1
12 Months or Longer, Unrealized Losses	(1.1)	(1.2)
Total, Fair Value	210.6	42.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	8.7	3.3
Preferred Stock Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	85.4	75.4
Available-for-sale Equity Securities, Gross Unrealized Gain	2.8	3.9
Available-for-sale Equity Securities, Gross Unrealized Loss	2.4	0.1
Available for sale Securities, Equity Securities, Fair Value	85.8	79.2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	22.7	0
Less Than 12 Months, Unrealized Losses	(2.3)	0
12 Months or Longer, Fair Value	2.4	2.4
12 Months or Longer, Unrealized Losses	(0.1)	(0.1)
Total, Fair Value	25.1	2.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	2.4	0.1
Preferred Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	13.4	18.4
Available-for-sale Equity Securities, Gross Unrealized Gain	4.6	3.8
Available-for-sale Equity Securities, Gross Unrealized Loss	0	0.9
Available for sale Securities, Equity Securities, Fair Value	18.0	21.3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0	2.3
Less Than 12 Months, Unrealized Losses	0	(0.8)
12 Months or Longer, Fair Value	0.7	3.7
12 Months or Longer, Unrealized Losses	0	(0.1)
Total, Fair Value	0.7	6.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	0.9
Common Stocks Manufacturing [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	68.1	67.0
Available-for-sale Equity Securities, Gross Unrealized Gain	29.2	20.9
Available-for-sale Equity Securities, Gross Unrealized Loss	0.2	0.4
Available for sale Securities, Equity Securities, Fair Value	97.1	87.5
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0.4	6.3
Less Than 12 Months, Unrealized Losses	(0.2)	(0.4)
12 Months or Longer, Fair Value	0.2	0
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	0.6	6.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0.2	0.4
Common Stocks Other Industries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	60.3	59.1
Available-for-sale Equity Securities, Gross Unrealized Gain	17.1	8.1
Available-for-sale Equity Securities, Gross Unrealized Loss	0.9	0.5
Available for sale Securities, Equity Securities, Fair Value	76.5	66.7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	11.7	14.2
Less Than 12 Months, Unrealized Losses	(0.8)	(0.4)
12 Months or Longer, Fair Value	0.5	1.3
12 Months or Longer, Unrealized Losses	(0.1)	(0.1)
Total, Fair Value	12.2	15.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0.9	0.5
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	137.1	119.6
Available-for-sale Equity Securities, Gross Unrealized Gain	0.8	6.3
Available-for-sale Equity Securities, Gross Unrealized Loss	1.9	0
Available for sale Securities, Equity Securities, Fair Value	136.0	125.9
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	106.8
Less Than 12 Months, Unrealized Losses	(1.9)
12 Months or Longer, Fair Value	0
12 Months or Longer, Unrealized Losses	0
Total, Fair Value	106.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	1.9
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	137.4	123.2
Available-for-sale Equity Securities, Gross Unrealized Gain	19.9	19.5
Available-for-sale Equity Securities, Gross Unrealized Loss	3.3	1.4
Available for sale Securities, Equity Securities, Fair Value	154.0	141.3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	59.5	5.5
Less Than 12 Months, Unrealized Losses	(2.4)	(0.5)
12 Months or Longer, Fair Value	5.7	6.7
12 Months or Longer, Unrealized Losses	(0.9)	(0.9)
Total, Fair Value	65.2	12.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	3.3	1.4
External Credit Rating, Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	63.1	3.8
External Credit Rating, Non Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	5.7	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	5.00%	3.00%