Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	$ 635.8	$ 634.8
Less Investment Expenses	7.1	7.2
Less Investment Income on PFA Assets	4.0	4.1
Less Amortization of Tax Credit Partnerships	3.6	4.0
Net Investment Income	621.1	619.5
Fixed Income Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	589.9	595.5
Derivative [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	7.7	6.3
Mortgage Receivable [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	26.0	25.0
Policy Loans [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	3.7	3.6
Other Long-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	7.7	3.2
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment Income, Interest and Dividend	$ 0.8	$ 1.2