INCOME TAXES - Components of Income Tax Expense (Benefit) Applicable to Pretax Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Provision of Income Taxes [Line Items]
Current									$ 794	$ 806	$ 1,379
Deferred									(1,413)	335	(316)
Total income tax expense	$ 265	$ (1,303)	$ 265	$ 154	$ 276	$ 259	$ 292	$ 314	(619)	1,141	1,063
Aflac Japan
Reconciliation of Provision of Income Taxes [Line Items]
Current									822	737	771
Deferred									(28)	183	93
Total income tax expense									794	920	864
Aflac U.S.
Reconciliation of Provision of Income Taxes [Line Items]
Current									(28)	69	608
Deferred									(1,385)	152	(409)
Total income tax expense									$ (1,413)	$ 221	$ 199