Fair Value (Estimated Fair Value And The Carrying Value Of Each Financial Instrument Categorized by Fair Value Hierarchy) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Total fixed maturity securities-available-for-sale	$ 7,120.4	$ 6,769.5
Commercial mortgage loans, net	5,149.0	4,902.3
Separate account assets	5,071.8	4,593.5
Other policyholder funds	5,324.5	5,078.1
Short-term debt	1.1	251.2
Long-term debt	551.5	300.9
Level 1 [Member]
Total fixed maturity securities-available-for-sale	0	0
Commercial mortgage loans, net	0	0
Policy loans	0	0
Separate account assets	4,926.5	4,444.4
Short-term debt	0	0
Long-term debt	0	0
Level 2 [Member]
Total fixed maturity securities-available-for-sale	7,038.2	6,696.4
Commercial mortgage loans, net	0	0
Policy loans	0	0
Separate account assets	145.3	149.1
Short-term debt	0.7	263.7
Long-term debt	568.7	272.0
Level 3 [Member]
Total fixed maturity securities-available-for-sale	82.2	73.1
Commercial mortgage loans, net	5,838.5	5,450.9
Policy loans	2.5	2.9
Separate account assets	0	0
Short-term debt	0	0
Long-term debt	0	0
U.S. Government and Agency Bonds [Member] | Level 1 [Member]
Total fixed maturity securities-available-for-sale	0	0
U.S. Government and Agency Bonds [Member] | Level 2 [Member]
Total fixed maturity securities-available-for-sale	430.3	451.6
U.S. Government and Agency Bonds [Member] | Level 3 [Member]
Total fixed maturity securities-available-for-sale	0	0.5
U.S. State and Political Subdivision Bonds [Member] | Level 1 [Member]
Total fixed maturity securities-available-for-sale	0	0
U.S. State and Political Subdivision Bonds [Member] | Level 2 [Member]
Total fixed maturity securities-available-for-sale	174.8	177.4
U.S. State and Political Subdivision Bonds [Member] | Level 3 [Member]
Total fixed maturity securities-available-for-sale	1.7	1.4
Foreign Government Bonds [Member] | Level 1 [Member]
Total fixed maturity securities-available-for-sale	0	0
Foreign Government Bonds [Member] | Level 2 [Member]
Total fixed maturity securities-available-for-sale	72.3	72.1
Foreign Government Bonds [Member] | Level 3 [Member]
Total fixed maturity securities-available-for-sale	0	0
Corporate Bonds [Member] | Level 1 [Member]
Total fixed maturity securities-available-for-sale	0	0
Corporate Bonds [Member] | Level 2 [Member]
Total fixed maturity securities-available-for-sale	6,360.8	5,995.3
Corporate Bonds [Member] | Level 3 [Member]
Total fixed maturity securities-available-for-sale	66.7	64.0
S and P Five Hundred Index Options [Member] | Level 1 [Member]
Total fixed maturity securities-available-for-sale	0	0
S and P Five Hundred Index Options [Member] | Level 2 [Member]
Total fixed maturity securities-available-for-sale	0	0
S and P Five Hundred Index Options [Member] | Level 3 [Member]
Total fixed maturity securities-available-for-sale	13.8	7.2
Investment Type Contracts [Member] | Level 1 [Member]
Other policyholder funds	0	0
Investment Type Contracts [Member] | Level 2 [Member]
Other policyholder funds	0	0
Investment Type Contracts [Member] | Level 3 [Member]
Other policyholder funds	5,088.7	4,804.8
Index Based Interest Guarantees [Member] | Level 1 [Member]
Index-based interest guarantees	0	0
Index Based Interest Guarantees [Member] | Level 2 [Member]
Index-based interest guarantees	0	0
Index Based Interest Guarantees [Member] | Level 3 [Member]
Index-based interest guarantees	58.6	49.5
Estimate of Fair Value, Fair Value Disclosure [Member]
Total fixed maturity securities-available-for-sale	7,120.4	6,769.5
Commercial mortgage loans, net	5,838.5	5,450.9
Policy loans	2.5	2.9
Separate account assets	5,071.8	4,593.5
Short-term debt	0.7	263.7
Long-term debt	568.7	272.0
Estimate of Fair Value, Fair Value Disclosure [Member] | U.S. Government and Agency Bonds [Member]
Total fixed maturity securities-available-for-sale	430.3	452.1
Estimate of Fair Value, Fair Value Disclosure [Member] | U.S. State and Political Subdivision Bonds [Member]
Total fixed maturity securities-available-for-sale	176.5	178.8
Estimate of Fair Value, Fair Value Disclosure [Member] | Foreign Government Bonds [Member]
Total fixed maturity securities-available-for-sale	72.3	72.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Corporate Bonds [Member]
Total fixed maturity securities-available-for-sale	6,427.5	6,059.3
Estimate of Fair Value, Fair Value Disclosure [Member] | S and P Five Hundred Index Options [Member]
Total fixed maturity securities-available-for-sale	13.8	7.2
Estimate of Fair Value, Fair Value Disclosure [Member] | Investment Type Contracts [Member]
Other policyholder funds	5,088.7	4,804.8
Estimate of Fair Value, Fair Value Disclosure [Member] | Index Based Interest Guarantees [Member]
Index-based interest guarantees	58.6	49.5
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total fixed maturity securities-available-for-sale	7,120.4	6,769.5
Commercial mortgage loans, net	5,149.0	4,902.3
Policy loans	2.5	2.9
Separate account assets	5,071.8	4,593.5
Short-term debt	1.1	251.2
Long-term debt	551.5	300.9
Carrying (Reported) Amount, Fair Value Disclosure [Member] | U.S. Government and Agency Bonds [Member]
Total fixed maturity securities-available-for-sale	430.3	452.1
Carrying (Reported) Amount, Fair Value Disclosure [Member] | U.S. State and Political Subdivision Bonds [Member]
Total fixed maturity securities-available-for-sale	176.5	178.8
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Foreign Government Bonds [Member]
Total fixed maturity securities-available-for-sale	72.3	72.1
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Corporate Bonds [Member]
Total fixed maturity securities-available-for-sale	6,427.5	6,059.3
Carrying (Reported) Amount, Fair Value Disclosure [Member] | S and P Five Hundred Index Options [Member]
Total fixed maturity securities-available-for-sale	13.8	7.2
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Investment Type Contracts [Member]
Other policyholder funds	4,700.4	4,449.2
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Index Based Interest Guarantees [Member]
Index-based interest guarantees	$ 58.6	$ 49.5