Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 366.8	$ 361.3	$ 1,084.9	$ 1,070.3
Less: investment expense	13.8	13.1	40.2	38.4
Net investment income	353.0	348.2	1,044.7	1,031.9
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	305.0	306.4	911.2	904.7
Investments that did not produce net investment income	1.2		1.2		$ 0.0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.0	0.7	3.5	5.2
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	51.2	41.7	139.4	123.5
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.1	3.4	9.6	10.0
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.1	0.5	0.4
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 5.4	$ 9.0	$ 20.7	$ 26.5