Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 643.0	$ 640.6	$ 1,270.6	$ 1,276.4
Less Investment Expenses	6.9	6.8	14.9	13.9
Less Investment Income on PFA Assets	3.8	3.9	7.6	7.9
Less Amortization of Tax Credit Partnerships	3.2	3.8	6.5	7.4
Net Investment Income	629.1	626.1	1,241.6	1,247.2
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	589.6	597.2	1,170.7	1,187.1
Derivative
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	10.3	9.0	20.2	16.7
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	26.6	27.1	55.0	53.1
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.0	3.9	7.9	7.6
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	11.9	2.8	15.7	10.5
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 0.6	$ 0.6	$ 1.1	$ 1.4