Fair Value (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 679.6	$ 632.3
Securities lending collateral	1,253.1	408.5
Long-term receivable (reported with other assets)	0
Total assets	4,224.3	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	23.0	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	1,683.7	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	584.9	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	514.2	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	12.1	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	58.6	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	693.9	561.3
Long-term receivable (reported with other assets)	0
Total assets	18,985.6	17,885.5
Total liabilities	(9.5)	(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)	20.0	58.4
Derivatives excluding embedded options (reported with other liabilities)	(9.5)	(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	192.3	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	18,079.4	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	148.3	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	5,971.3	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	8,589.4	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	103.5	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	2,231.6	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	516.4	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	518.9	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)	32.2
Total assets	247.1	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	43.1	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	171.8	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	150.4	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	3.6	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	17.8	14.8
Total [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	679.6	632.3
Securities lending collateral	1,947.0	969.8
Long-term receivable (reported with other assets)	32.2
Total assets	23,457.0	20,931.7
Total liabilities	(9.5)	(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)	20.0	58.4
Derivatives excluding embedded options (reported with other liabilities)	(9.5)	(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	23.0	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	1,919.1	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	18,836.1	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	514.2	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	148.3	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	5,971.3	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	8,751.9	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	103.5	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	2,231.6	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	520.0	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	$ 595.3	$ 458.7