Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 353.9	$ 352.7	$ 718.1	$ 709.0
Less: investment expense	13.7	12.6	26.4	25.3
Net investment income	340.2	340.1	691.7	683.7
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	300.8	297.5	606.2	598.3
Investments that did not produce net investment income	2.6		2.6		$ 0.0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	(0.1)	2.4	1.5	4.5
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	42.7	41.7	88.2	81.8
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.3	3.3	6.5	6.6
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.2	0.2	0.4	0.3
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 7.0	$ 7.6	$ 15.3	$ 17.5