Fair Value of Financial Instruments - Level 3 Assets and Liabilities by Pricing Source (Details) (Level 3, Fair Value, Measurements, Nonrecurring, USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012
Embedded derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	$ 103.5	[1]			$ 74.8	[2]
Total liabilities External	0	[3]			0	[3]
Total liabilities	103.5				74.8
Total liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities Internal	103.5	[1]			74.8	[2]
Total liabilities External	0	[3]			0	[3]
Total liabilities	103.5				74.8
U.S. government and agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	64.3	[1],[4]			58.8	[2],[4]
Total assets External	0	[3],[4]			0	[3],[4]
Total assets	64.3	[4]	65.8	[4]	58.8	[4]	47.1	[4]	31.4	[4]	25.9	[4]
State and political subdivision
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	48.5	[1]			45.4	[2]
Total assets External	96.1	[3]			88.4	[3]
Total assets	144.6		137.8		133.8		104.3		98.2		116.1
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	[1]			0	[2]
Total assets External	3.5	[3]			3.3	[3]
Total assets	3.5		3.4		3.3		3.3		9.5		8.0
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	962.4	[1]			874.1	[2]
Total assets External	182.7	[3]			228.2	[3]
Total assets	1,145.1		1,122.3		1,102.3		955.9		903.2		801.5
Commercial mortgage-backed (“CMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	[1]			0	[2]
Total assets External	2.9	[3]			31.9	[3]
Total assets	2.9		6.4		31.9		33.5		18.7		19.3
Residential mortgage-backed (“RMBS”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	[1]			0	[2]
Total assets External	161.0	[3]			175.9	[3]
Total assets	161.0		170.9		175.9		202.5		214.3		222.6
Collateralized debt obligations (“CDO”) / collateralized loan obligations (“CLO”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0	[1]			0	[2]
Total assets External	96.9	[3]			70.9	[3]
Total assets	96.9		86.2		70.9		67.6		62.2		56.8
Available-for-sale debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	10.3	[1]			11.3	[2]
Total assets External	64.4	[3]			71.0	[3]
Total assets	74.7		78.9		82.3		73.9		79.9		80.1
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,085.5	[1]			989.6	[2]
Total assets External	607.5	[3]			669.6	[3]
Total assets	1,693.0		1,671.7		1,659.2		1,488.1		1,417.4		1,330.3
Fair value investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	0.9	[1]			0.8	[2]
Total assets External	33.7	[3]			34.8	[3]
Total assets	34.6		35.2		35.6		34.4		31.4		22.7
Total assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets Internal	1,086.4	[1]			990.4	[2]
Total assets External	641.2	[3]			704.4	[3]
Total assets	$ 1,727.6		$ 1,706.9		$ 1,694.8		$ 1,522.5		$ 1,448.8		$ 1,353.0

[1] Excludes Level 3 assets which are valued based upon non-binding independent third-party valuations or third-party price information for which unobservable inputs are not reasonably available to us.

[2] Represents valuations reflecting both internally-derived and market inputs, as well as third-party information or quotes.

[3] Represents unadjusted prices from independent pricing services, third-party financial statements and independent indicative broker quotes where pricing inputs are not readily available.

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