Investments - Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 1,394.9	$ 1,465.9	$ 1,383.8
Less: investment expense	46.1	45.0	41.5
Net investment income	1,348.8	1,420.9	1,342.3
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1,222.5	1,224.2	1,182.4
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	7.5	13.6	15.3
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	143.5	118.1	104.0
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	13.2	13.7	13.3
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.4	0.8	0.8
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 6.8	$ 95.5	$ 68.0