Fair Value (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 612.4	$ 632.3
Securities lending collateral	532.0	408.5
Long-term receivable (reported with other assets)	0
Total assets	3,481.4	2,868.3
Total liabilities	0	0
Level I [Member] | Other Derivative Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	0	0
Derivatives excluding embedded options (reported with other liabilities)	0	0
Level I [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	25.9	16.3
Level I [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	1,766.2	1,475.7
Level I [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	544.9	335.5
Level I [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	461.6	299.9
Level I [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level I [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level I [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	12.0	0
Level I [Member] | Fixed Maturities [Member] | Options Embedded In Convertible Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level I [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level I [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level I [Member] | Fixed Maturities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	71.3	35.6
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Securities lending collateral	793.0	561.3
Long-term receivable (reported with other assets)	0
Total assets	18,836.7	17,885.5
Total liabilities	(20.9)	(20.7)
Level II [Member] | Other Derivative Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	29.0	58.4
Derivatives excluding embedded options (reported with other liabilities)	(20.9)	(20.7)
Level II [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	0	0
Level II [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	185.1	249.7
Level II [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	17,829.6	17,016.1
Level II [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level II [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	168.2	173.5
Level II [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	6,003.5	6,002.7
Level II [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	8,375.9	7,593.4
Level II [Member] | Fixed Maturities [Member] | Options Embedded In Convertible Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	97.6	89.2
Level II [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	2,181.6	2,268.9
Level II [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	516.1	480.1
Level II [Member] | Fixed Maturities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	486.7	408.3
Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Securities lending collateral	0	0
Long-term receivable (reported with other assets)	21.6
Total assets	196.7	177.9
Total liabilities	0	0
Level III [Member] | Other Derivative Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	0	0
Derivatives excluding embedded options (reported with other liabilities)	0	0
Level III [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	0	0
Level III [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	43.2	41.4
Level III [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	131.9	136.5
Level III [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level III [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level III [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level III [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	114.0	115.2
Level III [Member] | Fixed Maturities [Member] | Options Embedded In Convertible Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level III [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	0	0
Level III [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	3.1	6.5
Level III [Member] | Fixed Maturities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	14.8	14.8
Total [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	612.4	632.3
Securities lending collateral	1,325.0	969.8
Long-term receivable (reported with other assets)	21.6
Total assets	22,514.8	20,931.7
Total liabilities	(20.9)	(20.7)
Total [Member] | Other Derivative Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	29.0	58.4
Derivatives excluding embedded options (reported with other liabilities)	(20.9)	(20.7)
Total [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	25.9	16.3
Total [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	1,994.5	1,766.8
Total [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	18,506.4	17,488.1
Total [Member] | Fixed Maturities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	461.6	299.9
Total [Member] | Fixed Maturities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	168.2	173.5
Total [Member] | Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	6,003.5	6,002.7
Total [Member] | Fixed Maturities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	8,501.9	7,708.6
Total [Member] | Fixed Maturities [Member] | Options Embedded In Convertible Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	97.6	89.2
Total [Member] | Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	2,181.6	2,268.9
Total [Member] | Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	519.2	486.6
Total [Member] | Fixed Maturities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets measured on recurring basis available-for-sale securities	$ 572.8	$ 458.7