Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 394.7	$ 388.5	$ 790.7	$ 759.1
Less: investment expense	16.3	15.8	33.8	29.9
Net investment income	378.4	372.7	756.9	729.2
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	324.7	327.2	650.7	651.2
Investments that did not produce net investment income	1.3		1.3		$ 3.0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.5	1.4	2.8	3.0
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	52.6	51.6	103.8	94.6
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.9	3.0	5.8	6.0
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.2	0.5	0.5	0.9
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 12.8	$ 4.8	$ 27.1	$ 3.4