Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									$ 2,574.3	$ 2,578.3	$ 2,545.8
Less Investment Expenses									26.9	26.9	28.2
Less Investment Income on PFA Assets									16.1	17.4	18.8
Less Amortization of Tax Credit Partnerships									16.1	14.4	3.3
Net Investment Income	643.0	619.2	633.5	619.5	634.6	629.2	637.1	618.7	2,515.2	2,519.6	2,495.5
Fixed Income Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									2,404.0	2,425.2	2,401.9
Derivative [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									28.9	22.9	17.2
Mortgage Receivable [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									107.1	100.1	91.1
Policy Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									14.8	14.1	13.9
Other Long-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									15.2	13.1	18.5
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend									$ 4.3	$ 2.9	$ 3.2