Investments - Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 361.3	$ 340.2	$ 1,070.3	$ 1,055.7
Less: investment expense	13.1	12.0	38.4	36.4
Net investment income	348.2	328.2	1,031.9	1,019.3
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	306.4	292.2	904.7	900.5
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.7	1.7	5.2	1.6
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	41.7	39.0	123.5	116.5
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.4	3.2	10.0	9.6
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.2	0.4	0.7
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 9.0	$ 3.9	$ 26.5	$ 26.8