Fair Value of Financial Instruments - Fair Value by Level (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 3,840.9		$ 3,426.3
Assets
Separate account assets	1,951.0		2,052.7
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,840.9		3,426.3
Short-term Investments, Fair Value Disclosure	189.8		81.0
Derivative assets	170.0		237.8
Assets
Fair value investments	47.6		48.6
Separate account assets	1,951.0		2,052.7
Total assets	6,199.3		5,846.4
Liabilities
Derivative liabilities	85.0		110.8
Embedded derivatives	103.5		74.8
Total liabilities	188.5		185.6
Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Short-term Investments, Fair Value Disclosure	189.8		80.0
Derivative assets	10.8		12.5
Assets
Fair value investments	0		0
Separate account assets	1,951.0		2,052.7
Total assets	2,151.6		2,145.2
Liabilities
Derivative liabilities	0		0
Embedded derivatives	0		0
Total liabilities	0		0
Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,147.9		1,767.1
Short-term Investments, Fair Value Disclosure	0		1.0
Derivative assets	159.2		225.3
Assets
Fair value investments	13.0		13.0
Separate account assets	0		0
Total assets	2,320.1		2,006.4
Liabilities
Derivative liabilities	85.0		110.8
Embedded derivatives	0		0
Total liabilities	85.0		110.8
Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,693.0		1,659.2
Short-term Investments, Fair Value Disclosure	0		0
Derivative assets	0		0
Assets
Fair value investments	34.6		35.6
Separate account assets	0		0
Total assets	1,727.6		1,694.8
Liabilities
Derivative liabilities	0		0
Embedded derivatives	103.5		74.8
Total liabilities	103.5		74.8
U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	73.5		68.6
U.S. government and agency | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	73.5	[1]	68.6	[1]
U.S. government and agency | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	[1]	0	[1]
U.S. government and agency | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	9.2	[1]	9.8	[1]
U.S. government and agency | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	64.3	[1]	58.8	[1]
State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	172.1		154.6
State and political subdivision | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	172.1		154.6
State and political subdivision | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
State and political subdivision | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27.5		20.8
State and political subdivision | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	144.6		133.8
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	70.2		65.5
Foreign government | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	70.2		65.5
Foreign government | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Foreign government | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	66.7		62.2
Foreign government | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3.5		3.3
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,503.2		2,207.3
Corporate | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,503.2		2,207.3
Corporate | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Corporate | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,358.1		1,105.0
Corporate | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,145.1		1,102.3
Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	259.9		253.7
Commercial mortgage-backed (“CMBS”) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	259.9		253.7
Commercial mortgage-backed (“CMBS”) | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Commercial mortgage-backed (“CMBS”) | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	257.0		221.8
Commercial mortgage-backed (“CMBS”) | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2.9		31.9
Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	579.5		509.8
Residential mortgage-backed (“RMBS”) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	579.5		509.8
Residential mortgage-backed (“RMBS”) | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Residential mortgage-backed (“RMBS”) | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	418.5		333.9
Residential mortgage-backed (“RMBS”) | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	161.0		175.9
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	96.9		70.9
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	96.9		70.9
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”) | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”) | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”) | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	96.9		70.9
Available-for-sale debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	85.6		95.9
Available-for-sale debt securities | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	85.6		95.9
Available-for-sale debt securities | Fair Value, Measurements, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0		0
Available-for-sale debt securities | Fair Value, Measurements, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10.9		13.6
Available-for-sale debt securities | Fair Value, Measurements, Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 74.7		$ 82.3

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