Investments Components of Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net Investment Income [Line Items]
Investment income	$ 99.2	$ 102.2	$ 202.5	$ 202.7
Investment expenses	(6.0)	(4.4)	(10.1)	(9.3)
Net Investment Income	93.2	97.8	192.4	193.4
Fixed maturities
Net Investment Income [Line Items]
Investment income	78.4	80.8	161.2	161.7
Fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Investment income	12.0	12.1	25.0	23.9
Fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Investment income	12.6	11.1	25.4	22.5
Fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Investment income	0.1	0	0.2	0
Fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Investment income	18.8	24.2	40.8	49.8
Fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Investment income	11.3	5.7	21.2	10.2
Fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Investment income	16.0	18.1	32.7	36.8
Fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Investment income	3.7	4.2	7.9	7.7
Fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Investment income	3.9	5.4	8.0	10.8
Equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Investment income	9.3	8.9	19.2	18.1
Equity securities | Common equities
Net Investment Income [Line Items]
Investment income	11.2	11.8	21.6	21.5
Short-term investments | Other short-term investments
Net Investment Income [Line Items]
Investment income	$ 0.3	$ 0.7	$ 0.5	$ 1.4