Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 396.6	$ 366.8	$ 1,155.7	$ 1,084.9
Less: investment expense	15.9	13.8	45.8	40.2
Net investment income	380.7	353.0	1,109.9	1,044.7
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	335.0	305.0	986.2	911.2
Investments that did not produce net investment income	3.0		3.0		$ 1.1
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.1	2.0	4.1	3.5
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	49.5	51.2	144.1	139.4
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.0	3.1	9.0	9.6
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.1	1.0	0.5
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 7.9	$ 5.4	$ 11.3	$ 20.7