Investments - Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 356.3	$ 360.2
Less: investment expense	12.7	12.0
Net investment income	343.6	348.2
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	300.8	307.6
Investments in fixed maturities not producing income	0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.1	0.9
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	40.1	36.6
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.3	3.2
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.2
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 9.9	$ 11.7