Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fixed Maturity Securities
Gross Gains on Sales									$ 13.8	$ 34.3	$ 23.8
Gross Losses on Sales									(4.4)	(17.2)	(25.6)
Other-Than-Temporary Impairment Loss									(8.1)	(30.5)	(32.4)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales									9.4	5.5	16.0
Gross Losses on Sales									(0.2)	(0.7)	(0.1)
Impairment Loss									(0.9)	(5.7)	(5.9)
Gain (Loss) on Embedded Derivative									30.8	40.9	(37.7)
All Other Derivatives									(0.1)	(1.5)	35.7
Foreign Currency Transactions									0.0	(0.9)	(17.6)
Net Realized Investment Gain (Loss)	$ 11.4	$ 9.8	$ 8.1	$ 11.0	$ 28.4	$ 11.0	$ 5.3	$ (20.5)	$ 40.3	$ 24.2	$ (43.8)