Fair Value of Financial Instruments - Fair Value by Level (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Assets
Fair Value	$ 3,589.1		$ 3,426.3
Separate account assets	1,979.3		2,052.7
U.S. government and agency
Assets
Fair Value	75.1		68.6
State and political subdivision
Assets
Fair Value	162.5		154.6
Foreign government
Assets
Fair Value	68.3		65.5
Corporate
Assets
Fair Value	2,348.9		2,207.3
Commercial mortgage-backed (“CMBS”)
Assets
Fair Value	257.6		253.7
Residential mortgage-backed (“RMBS”)
Assets
Fair Value	502.6		509.8
Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Assets
Fair Value	86.2		70.9
Other asset-backed
Assets
Fair Value	87.9		95.9
Fair Value, Measurements, Nonrecurring
Assets
Fair Value	3,589.1		3,426.3
Short-term Investments, Fair Value Disclosure	190.9		81.0
Derivative assets	167.6		237.8
Fair value investments	48.2		48.6
Separate account assets	1,979.3		2,052.7
Total assets	5,975.1		5,846.4
Liabilities
Derivative liabilities	84.6		110.8
Embedded derivatives	93.7		74.8
Total liabilities	178.3		185.6
Fair Value, Measurements, Nonrecurring | Level 1
Assets
Fair Value	0		0
Short-term Investments, Fair Value Disclosure	190.9		80.0
Derivative assets	11.0		12.5
Fair value investments	0		0
Separate account assets	1,979.3		2,052.7
Total assets	2,181.2		2,145.2
Liabilities
Derivative liabilities	0		0
Embedded derivatives	0		0
Total liabilities	0		0
Fair Value, Measurements, Nonrecurring | Level 2
Assets
Fair Value	1,917.4		1,767.1
Short-term Investments, Fair Value Disclosure	0		1.0
Derivative assets	156.6		225.3
Fair value investments	13.0		13.0
Separate account assets	0		0
Total assets	2,087.0		2,006.4
Liabilities
Derivative liabilities	84.6		110.8
Embedded derivatives	0		0
Total liabilities	84.6		110.8
Fair Value, Measurements, Nonrecurring | Level 3
Assets
Fair Value	1,671.7		1,659.2
Short-term Investments, Fair Value Disclosure	0		0
Derivative assets	0		0
Fair value investments	35.2		35.6
Separate account assets	0		0
Total assets	1,706.9		1,694.8
Liabilities
Derivative liabilities	0		0
Embedded derivatives	93.7		74.8
Total liabilities	93.7		74.8
Fair Value, Measurements, Nonrecurring | U.S. government and agency
Assets
Fair Value	75.1	[1]	68.6	[1]
Fair Value, Measurements, Nonrecurring | U.S. government and agency | Level 1
Assets
Fair Value	0	[1]	0	[1]
Fair Value, Measurements, Nonrecurring | U.S. government and agency | Level 2
Assets
Fair Value	9.3	[1]	9.8	[1]
Fair Value, Measurements, Nonrecurring | U.S. government and agency | Level 3
Assets
Fair Value	65.8	[1]	58.8	[1]
Fair Value, Measurements, Nonrecurring | State and political subdivision
Assets
Fair Value	162.5		154.6
Fair Value, Measurements, Nonrecurring | State and political subdivision | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | State and political subdivision | Level 2
Assets
Fair Value	24.7		20.8
Fair Value, Measurements, Nonrecurring | State and political subdivision | Level 3
Assets
Fair Value	137.8		133.8
Fair Value, Measurements, Nonrecurring | Foreign government
Assets
Fair Value	68.3		65.5
Fair Value, Measurements, Nonrecurring | Foreign government | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Foreign government | Level 2
Assets
Fair Value	64.9		62.2
Fair Value, Measurements, Nonrecurring | Foreign government | Level 3
Assets
Fair Value	3.4		3.3
Fair Value, Measurements, Nonrecurring | Corporate
Assets
Fair Value	2,348.9		2,207.3
Fair Value, Measurements, Nonrecurring | Corporate | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Corporate | Level 2
Assets
Fair Value	1,226.6		1,105.0
Fair Value, Measurements, Nonrecurring | Corporate | Level 3
Assets
Fair Value	1,122.3		1,102.3
Fair Value, Measurements, Nonrecurring | Commercial mortgage-backed (“CMBS”)
Assets
Fair Value	257.6		253.7
Fair Value, Measurements, Nonrecurring | Commercial mortgage-backed (“CMBS”) | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Commercial mortgage-backed (“CMBS”) | Level 2
Assets
Fair Value	251.2		221.8
Fair Value, Measurements, Nonrecurring | Commercial mortgage-backed (“CMBS”) | Level 3
Assets
Fair Value	6.4		31.9
Fair Value, Measurements, Nonrecurring | Residential mortgage-backed (“RMBS”)
Assets
Fair Value	502.6		509.8
Fair Value, Measurements, Nonrecurring | Residential mortgage-backed (“RMBS”) | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Residential mortgage-backed (“RMBS”) | Level 2
Assets
Fair Value	331.7		333.9
Fair Value, Measurements, Nonrecurring | Residential mortgage-backed (“RMBS”) | Level 3
Assets
Fair Value	170.9		175.9
Fair Value, Measurements, Nonrecurring | Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”)
Assets
Fair Value	86.2		70.9
Fair Value, Measurements, Nonrecurring | Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”) | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”) | Level 2
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Collateralized debt obligation (“CDO”) / collateralized loan obligation (“CLO”) | Level 3
Assets
Fair Value	86.2		70.9
Fair Value, Measurements, Nonrecurring | Other asset-backed
Assets
Fair Value	87.9		95.9
Fair Value, Measurements, Nonrecurring | Other asset-backed | Level 1
Assets
Fair Value	0		0
Fair Value, Measurements, Nonrecurring | Other asset-backed | Level 2
Assets
Fair Value	9.0		13.6
Fair Value, Measurements, Nonrecurring | Other asset-backed | Level 3
Assets
Fair Value	$ 78.9		$ 82.3

[1] Level 3 includes securities whose underlying collateral is an obligation of a U.S. government entity.