Reconciliation of Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Insurance [Abstract]
Policy and Contract Benefits	$ 1,801.6	$ 1,745.5	$ 1,720.5
Reserves for Future Policy and Contract Benefits	48,844.3	47,780.1	48,060.8
Total Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits	50,645.9		49,781.3
Life Reserves for Future Policy and Contract Benefits	8,327.8		8,396.2
Accident and Health Active Life Reserves	12,478.5		12,092.9
Adjustment Related to Unrealized Investment Gains and Losses	6,971.3		6,334.2
Liability for Unpaid Claims and Claim Adjustment Expense	$ 22,868.3	$ 23,076.7	$ 22,958.0	$ 23,149.0