Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 627.6	$ 635.8
Less Investment Expenses	8.0	7.1
Less Investment Income on PFA Assets	3.8	4.0
Less Amortization of Tax Credit Partnerships	3.3	3.6
Net Investment Income	612.5	621.1
Fixed Income Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	581.1	589.9
Derivative
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	9.9	7.7
Mortgage Receivable
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	28.4	26.0
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	3.9	3.7
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	3.8	7.7
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 0.5	$ 0.8