Investments - 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	Dec. 31, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 352.7	$ 355.3	$ 709.0	$ 715.5
Less: investment expense	12.6	12.4	25.3	24.4
Net investment income	340.1	342.9	683.7	691.1
Investments in fixed maturities not producing income	0		0
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	297.5	300.7	598.3	608.3
Investments in fixed maturities not producing income					0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.4	(1.0)	4.5	(0.1)
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	41.7	40.9	81.8	77.5
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.3	3.2	6.6	6.4
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.2	0.3	0.3	0.5
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 7.6	$ 11.2	$ 17.5	$ 22.9