Investments Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 330.6	$ 374.5	$ 1,018.2	$ 1,129.3
Less: investment expenses	(13.1)	(15.6)	(39.5)	(44.5)
Net investment income	317.5	358.9	978.7	1,084.8
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	280.5	311.8	865.7	934.3
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.2	1.0	3.0	6.1
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	40.1	46.5	126.8	130.9
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1.5	1.7	4.4	5.0
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	(0.2)	0.4	0.1	1.8
Other investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 7.5	$ 13.1	$ 18.2	$ 51.2