Fair Value (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 701.0	$ 573.2
Available-for-sale securities	18,951.0	19,909.9
Other invested assets, current	19.1	20.2
Securities lending collateral	1,300.4	1,515.2
Total assets	21,130.6	22,243.3
Total liabilities	(259.4)	(260.4)
Fixed Maturity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	17,478.2	17,971.8
Fixed Maturity Securities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	349.9	320.0
Fixed Maturity Securities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	75.9	95.0
Fixed Maturity Securities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	6,251.6	5,733.6
Fixed Maturity Securities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	7,893.1	8,332.5
Fixed Maturity Securities [Member] | Options Embedded In Convertible Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		98.7
Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,750.9	2,159.0
Fixed Maturity Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	404.3	509.9
Fixed Maturity Securities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	752.5	723.1
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,472.8	1,938.1
Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	19.1	20.2
Derivatives Excluding Embedded Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	159.1	224.8
Derivatives (reported with other liabilities)	(259.4)	(260.4)
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	701.0	573.2
Securities lending collateral	708.1	808.3
Total assets	3,165.2	3,514.9
Total liabilities	0.0	0.0
Level I [Member] | Fixed Maturity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	483.2	416.3
Level I [Member] | Fixed Maturity Securities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	349.9	320.0
Level I [Member] | Fixed Maturity Securities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level I [Member] | Fixed Maturity Securities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level I [Member] | Fixed Maturity Securities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	77.6	7.1
Level I [Member] | Fixed Maturity Securities [Member] | Options Embedded In Convertible Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0.0
Level I [Member] | Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level I [Member] | Fixed Maturity Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level I [Member] | Fixed Maturity Securities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	55.7	89.2
Level I [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,253.8	1,696.9
Level I [Member] | Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	19.1	20.2
Level I [Member] | Derivatives Excluding Embedded Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	0.0	0.0
Derivatives (reported with other liabilities)	0.0	0.0
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0.0	0.0
Securities lending collateral	592.3	706.9
Total assets	17,649.6	18,525.6
Total liabilities	(259.4)	(260.4)
Level II [Member] | Fixed Maturity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	16,781.3	17,401.0
Level II [Member] | Fixed Maturity Securities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level II [Member] | Fixed Maturity Securities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	75.9	95.0
Level II [Member] | Fixed Maturity Securities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	6,251.6	5,733.6
Level II [Member] | Fixed Maturity Securities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	7,629.3	8,180.8
Level II [Member] | Fixed Maturity Securities [Member] | Options Embedded In Convertible Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		98.7
Level II [Member] | Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,750.9	2,159.0
Level II [Member] | Fixed Maturity Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	402.4	506.6
Level II [Member] | Fixed Maturity Securities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	671.2	627.3
Level II [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	116.9	192.9
Level II [Member] | Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	0.0	0.0
Level II [Member] | Derivatives Excluding Embedded Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	159.1	224.8
Derivatives (reported with other liabilities)	(259.4)	(260.4)
Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0.0	0.0
Securities lending collateral	0.0	0.0
Total assets	315.8	202.8
Total liabilities	0.0	0.0
Level III [Member] | Fixed Maturity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	213.7	154.5
Level III [Member] | Fixed Maturity Securities [Member] | United States Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level III [Member] | Fixed Maturity Securities [Member] | Government Sponsored Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level III [Member] | Fixed Maturity Securities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level III [Member] | Fixed Maturity Securities [Member] | Corporate Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	186.2	144.6
Level III [Member] | Fixed Maturity Securities [Member] | Options Embedded In Convertible Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0.0
Level III [Member] | Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0.0	0.0
Level III [Member] | Fixed Maturity Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1.9	3.3
Level III [Member] | Fixed Maturity Securities [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	25.6	6.6
Level III [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	102.1	48.3
Level III [Member] | Other Invested Assets, Current [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, current	0.0	0.0
Level III [Member] | Derivatives Excluding Embedded Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives excluding embedded options (reported with other assets)	0.0	0.0
Derivatives (reported with other liabilities)	$ 0.0	$ 0.0