Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 622.1	$ 630.1	$ 1,892.7	$ 1,906.5
Less Investment Expenses	7.0	7.2	21.9	21.1
Less Investment Income on PFA Assets	3.7	3.9	11.3	11.8
Less Amortization of Tax Credit Partnerships	5.0	3.5	11.5	10.9
Net Investment Income	606.4	615.5	1,848.0	1,862.7
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	578.8	584.2	1,749.5	1,771.3
Derivative
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	10.1	8.9	30.3	25.6
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	25.8	28.5	80.8	81.6
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.2	4.0	12.1	11.6
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	2.6	4.0	18.3	14.5
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 0.6	$ 0.5	$ 1.7	$ 1.9