Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									$ 2,578.3	$ 2,545.8	$ 2,393.8
Less Investment Expenses									26.9	28.2	29.2
Less Investment Income on PFA Assets									17.4	18.8	18.0
Less Amortization of Tax Credit Partnerships									14.4	3.3	0
Net Investment Income	634.6	629.2	637.1	618.7	634.3	618.4	629.8	613.0	2,519.6	2,495.5	2,346.6
Fixed Income Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									2,425.2	2,401.9	2,268.5
Derivative [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									22.9	17.2	13.5
Mortgage Receivable [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									100.1	91.1	81.0
Policy Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									14.1	13.9	12.4
Other Long-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									13.1	18.5	11.5
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									$ 2.9	$ 3.2	$ 6.9