Fair Value Measurements, Recurring and Nonrecurring (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
ASSETS:
Available-for-sale debt securities, at fair value	$ 3,426.3		$ 3,252.1	$ 3,162.2	$ 3,054.7	$ 2,972.3
Separate account assets	2,052.7		2,049.5	2,021.3	2,109.5	2,061.8
U.S. government and agency
ASSETS:
Available-for-sale debt securities, at fair value	68.6					40.2
State and political subdivision
ASSETS:
Available-for-sale debt securities, at fair value	154.6					130.9
Foreign government
ASSETS:
Available-for-sale debt securities, at fair value	65.5					51.8
Corporate
ASSETS:
Available-for-sale debt securities, at fair value	2,207.3					1,897.5
Commercial mortgage-backed (“CMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	253.7					254.0
Residential mortgage-backed (“RMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	509.8					422.5
CDO/CLO
ASSETS:
Available-for-sale debt securities, at fair value	70.9					56.8
Other asset-backed
ASSETS:
Available-for-sale debt securities, at fair value	95.9					118.6
Level 1 | CDO/CLO
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Level 2 | CDO/CLO
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Level 3 | CDO/CLO
ASSETS:
Available-for-sale debt securities, at fair value	70.9					56.8
Nonrecurring
ASSETS:
Available-for-sale debt securities, at fair value	3,426.3					2,972.3
Short-term investments	81.0					244.9
Derivative assets	237.8					149.4
Related party reinsurance derivative asset	0					0
Fair value investments	48.6					38.5
Separate account assets	2,052.7					2,061.8
Total assets	5,846.4					5,466.9
Liabilities
Derivative liabilities	110.8					45.7
Embedded derivatives	74.8					79.5
Total liabilities	185.6					125.2
Nonrecurring | U.S. government and agency
ASSETS:
Available-for-sale debt securities, at fair value	68.6					40.2
Nonrecurring | State and political subdivision
ASSETS:
Available-for-sale debt securities, at fair value	154.6					130.9
Nonrecurring | Foreign government
ASSETS:
Available-for-sale debt securities, at fair value	65.5					51.8
Nonrecurring | Corporate
ASSETS:
Available-for-sale debt securities, at fair value	2,207.3					1,897.5
Nonrecurring | Commercial mortgage-backed (“CMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	253.7					254.0
Nonrecurring | Residential mortgage-backed (“RMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	509.8					422.5
Nonrecurring | Other asset-backed
ASSETS:
Available-for-sale debt securities, at fair value	95.9					118.6
Nonrecurring | Level 1
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Short-term investments	80.0					244.9
Derivative assets	12.5					13.8
Related party reinsurance derivative asset	0					0
Fair value investments	0					0
Separate account assets	2,052.7					2,061.8
Total assets	2,145.2					2,320.5
Liabilities
Derivative liabilities	0					0
Embedded derivatives	0					0
Total liabilities	0					0
Nonrecurring | Level 1 | U.S. government and agency
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 1 | State and political subdivision
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 1 | Foreign government
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 1 | Corporate
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 1 | Commercial mortgage-backed (“CMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 1 | Residential mortgage-backed (“RMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 1 | Other asset-backed
ASSETS:
Available-for-sale debt securities, at fair value	0					0
Nonrecurring | Level 2
ASSETS:
Available-for-sale debt securities, at fair value	1,767.1					1,642.0
Short-term investments	1.0					0
Derivative assets	225.3					135.6
Related party reinsurance derivative asset	0					0
Fair value investments	13.0					15.8
Separate account assets	0					0
Total assets	2,006.4					1,793.4
Liabilities
Derivative liabilities	110.8					45.7
Embedded derivatives	0					0
Total liabilities	110.8					45.7
Nonrecurring | Level 2 | U.S. government and agency
ASSETS:
Available-for-sale debt securities, at fair value	9.8					14.3
Nonrecurring | Level 2 | State and political subdivision
ASSETS:
Available-for-sale debt securities, at fair value	20.8					14.8
Nonrecurring | Level 2 | Foreign government
ASSETS:
Available-for-sale debt securities, at fair value	62.2					43.8
Nonrecurring | Level 2 | Corporate
ASSETS:
Available-for-sale debt securities, at fair value	1,105.0					1,096.0
Nonrecurring | Level 2 | Commercial mortgage-backed (“CMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	221.8					234.7
Nonrecurring | Level 2 | Residential mortgage-backed (“RMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	333.9					199.9
Nonrecurring | Level 2 | Other asset-backed
ASSETS:
Available-for-sale debt securities, at fair value	13.6					38.5
Nonrecurring | Level 3
ASSETS:
Available-for-sale debt securities, at fair value	1,659.2					1,330.3
Short-term investments	0					0
Derivative assets	0					0
Related party reinsurance derivative asset	0					0
Fair value investments	35.6					22.7
Separate account assets	0					0
Total assets	1,694.8					1,353.0
Liabilities
Derivative liabilities	0					0
Embedded derivatives	74.8					79.5
Total liabilities	74.8					79.5
Nonrecurring | Level 3 | U.S. government and agency
ASSETS:
Available-for-sale debt securities, at fair value	58.8	[1]				25.9	[1]
Nonrecurring | Level 3 | State and political subdivision
ASSETS:
Available-for-sale debt securities, at fair value	133.8					116.1
Nonrecurring | Level 3 | Foreign government
ASSETS:
Available-for-sale debt securities, at fair value	3.3					8.0
Nonrecurring | Level 3 | Corporate
ASSETS:
Available-for-sale debt securities, at fair value	1,102.3					801.5
Nonrecurring | Level 3 | Commercial mortgage-backed (“CMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	31.9					19.3
Nonrecurring | Level 3 | Residential mortgage-backed (“RMBS”)
ASSETS:
Available-for-sale debt securities, at fair value	175.9					222.6
Nonrecurring | Level 3 | Other asset-backed
ASSETS:
Available-for-sale debt securities, at fair value	$ 82.3					$ 80.1

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