Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	$ 630.1	$ 633.7	$ 1,906.5	$ 1,916.6
Less Investment Expenses	7.2	6.3	21.1	20.3
Less Investment Income on PFA Assets	3.9	4.1	11.8	12.1
Less Amortization of Tax Credit Partnerships	3.5	4.1	10.9	12.0
Net Investment Income	615.5	619.2	1,862.7	1,872.2
Fixed Income Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	584.2	589.0	1,771.3	1,790.8
Derivative [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	8.9	9.1	25.6	22.0
Mortgage Receivable [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	28.5	25.5	81.6	76.5
Policy Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	4.0	3.7	11.6	10.8
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	4.0	5.4	14.5	13.2
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	$ 0.5	$ 1.0	$ 1.9	$ 3.3