Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 388.5	$ 353.9	$ 759.1	$ 718.1
Less: investment expense	15.8	13.7	29.9	26.4
Net investment income	372.7	340.2	729.2	691.7
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	327.2	300.8	651.2	606.2
Investments that did not produce net investment income	0.5		0.5		$ 1.1
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.4	(0.1)	3.0	1.5
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	51.6	42.7	94.6	88.2
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.0	3.3	6.0	6.5
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.5	0.2	0.9	0.4
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 4.8	$ 7.0	$ 3.4	$ 15.3