Investments - Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 360.2	$ 363.2
Less: investment expense	12.0	11.2
Net investment income	348.2	352.0
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	307.6	305.8
Investments in fixed maturities not producing income	0		0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.9	1.1
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	36.6	33.6
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.2	3.3
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.2	0
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 11.7	$ 19.4