Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 396.3	$ 396.6	$ 1,187.0	$ 1,155.7
Less: investment expense	17.5	15.9	51.3	45.8
Net investment income	378.8	380.7	1,135.7	1,109.9
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	324.1	335.0	974.8	986.2
Investments that did not produce net investment income	1.1		1.1		$ 3.0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.0	1.1	3.8	4.1
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	53.2	49.5	157.0	144.1
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.4	3.0	8.2	9.0
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.2	0.1	0.7	1.0
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 15.4	$ 7.9	$ 42.5	$ 11.3