Investments Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Investment Income
Investment Income, Interest and Dividend									$ 2,527.0	$ 2,536.3	$ 2,552.1
Less Investment Expenses									31.2	29.0	29.5
Less Investment Income on PFA Assets									14.6	15.1	15.7
Net Investment Income	$ 636.4	$ 612.1	$ 630.7	$ 602.0	$ 632.7	$ 611.3	$ 632.4	$ 615.8	2,481.2	2,492.2	2,506.9
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend									2,327.1	2,344.4	2,371.6
Derivatives
Net Investment Income
Investment Income, Interest and Dividend									44.2	40.4	35.2
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend									114.0	109.8	109.2
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend									16.7	16.3	15.7
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend									21.6	23.0	18.0
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend									$ 3.4	$ 2.4	$ 2.4