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	$ 355.7	$ 348.6	$ 1,042.6	$ 1,093.8
Less: investment expense	(11.0)	(11.9)	(34.0)	(33.4)
Net investment income	344.7	336.7	1,008.6	1,060.4
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	308.2	297.8	916.2	910.4
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.6	3.2	6.7	11.7
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	35.0	30.7	106.1	86.3
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.3	3.8	9.9	10.3
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.3	0.4	0.8	1.1
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 6.3	$ 12.7	$ 2.9	$ 74.0