Fair Value of Financial Instruments - Fair Value by Level (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Available-for-sale debt securities	$ 3,054.7	$ 2,972.3
Related party reinsurance derivative asset	0	0
Separate account assets	2,109.5	2,061.8
Level 1
Assets
Related party reinsurance derivative asset	0	0
Level 2
Assets
Related party reinsurance derivative asset	0	0
Level 3
Assets
Related party reinsurance derivative asset	0	0
Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	3,054.7	2,972.3
Short-term Investments, Fair Value Disclosure	224.9	244.9
Derivative assets	186.6	149.4
Fair value investments	46.5	38.5
Separate account assets	2,109.5	2,061.8
Total assets	5,622.2	5,466.9
Liabilities
Derivative liabilities	76.4	45.7
Embedded derivatives	81.8	79.5
Total liabilities	158.2	125.2
Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
Short-term Investments, Fair Value Disclosure	224.9	244.9
Derivative assets	10.8	13.8
Fair value investments	0	0
Separate account assets	2,109.5	2,061.8
Total assets	2,345.2	2,320.5
Liabilities
Derivative liabilities	0	0
Embedded derivatives	0	0
Total liabilities	0	0
Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	1,637.3	1,642.0
Short-term Investments, Fair Value Disclosure	0	0
Derivative assets	175.8	135.6
Fair value investments	15.1	15.8
Separate account assets	0	0
Total assets	1,828.2	1,793.4
Liabilities
Derivative liabilities	76.4	45.7
Embedded derivatives	0	0
Total liabilities	76.4	45.7
Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	1,417.4	1,330.3
Short-term Investments, Fair Value Disclosure	0	0
Derivative assets	0	0
Fair value investments	31.4	22.7
Separate account assets	0	0
Total assets	1,448.8	1,353.0
Liabilities
Derivative liabilities	0	0
Embedded derivatives	81.8	79.5
Total liabilities	81.8	79.5
U.S. government and agency
Assets
Available-for-sale debt securities	45.6	40.2
U.S. government and agency | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	45.6	40.2
U.S. government and agency | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
U.S. government and agency | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	14.2	14.3
U.S. government and agency | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	31.4	25.9
State and political subdivision
Assets
Available-for-sale debt securities	117.1	130.9
State and political subdivision | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	117.1	130.9
State and political subdivision | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
State and political subdivision | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	18.9	14.8
State and political subdivision | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	98.2	116.1
Foreign government
Assets
Available-for-sale debt securities	50.1	51.8
Foreign government | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	50.1	51.8
Foreign government | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
Foreign government | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	40.6	43.8
Foreign government | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	9.5	8.0
Corporate
Assets
Available-for-sale debt securities	2,025.5	1,897.5
Corporate | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	2,025.5	1,897.5
Corporate | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
Corporate | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	1,122.3	1,096.0
Corporate | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	903.2	801.5
Commercial mortgage-backed (CMBS)
Assets
Available-for-sale debt securities	248.7	254.0
Commercial mortgage-backed (CMBS) | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	248.7	254.0
Commercial mortgage-backed (CMBS) | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
Commercial mortgage-backed (CMBS) | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	230.0	234.7
Commercial mortgage-backed (CMBS) | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	18.7	19.3
Residential mortgage-backed (RMBS)
Assets
Available-for-sale debt securities	396.0	422.5
Residential mortgage-backed (RMBS) | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	396.0	422.5
Residential mortgage-backed (RMBS) | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
Residential mortgage-backed (RMBS) | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	181.7	199.9
Residential mortgage-backed (RMBS) | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	214.3	222.6
CDO/CLO
Assets
Available-for-sale debt securities	62.2	56.8
CDO/CLO | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	62.2	56.8
CDO/CLO | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
CDO/CLO | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	0	0
CDO/CLO | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	62.2	56.8
Other asset-backed
Assets
Available-for-sale debt securities	109.5	118.6
Other asset-backed | Fair Value, Measurements, Nonrecurring
Assets
Available-for-sale debt securities	109.5	118.6
Other asset-backed | Fair Value, Measurements, Nonrecurring | Level 1
Assets
Available-for-sale debt securities	0	0
Other asset-backed | Fair Value, Measurements, Nonrecurring | Level 2
Assets
Available-for-sale debt securities	29.6	38.5
Other asset-backed | Fair Value, Measurements, Nonrecurring | Level 3
Assets
Available-for-sale debt securities	$ 79.9	$ 80.1