Reconciliation of Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Insurance [Abstract]
Policy and Contract Benefits	$ 1,855.4	$ 1,745.5	$ 1,695.7
Reserves for Future Policy and Contract Benefits	49,653.0	47,780.1	44,841.9
Total Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits	51,508.4	49,525.6	46,537.6
Life Reserves for Future Policy and Contract Benefits	8,371.7	8,435.7	8,330.9
Accident and Health Active Life Reserves	12,730.9	12,210.1	11,837.4
Adjustment Related to Unrealized Investment Gains and Losses	6,225.6	5,803.1	3,220.3
Liability for Unpaid Claims and Claim Adjustment Expense	$ 24,180.2	$ 23,076.7	$ 23,149.0	$ 23,222.0