INCOME TAXES - Components of Income Tax Expense (Benefit) Applicable to Pretax Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Provision of Income Taxes [Line Items]
Current									$ 806	$ 1,379	$ 631
Deferred									335	(316)	(1,217)
Total income tax expense	$ 276	$ 259	$ 292	$ 314	$ 197	$ 301	$ 299	$ 265	1,141	1,063	(586)
Aflac Japan
Reconciliation of Provision of Income Taxes [Line Items]
Current									737	771	722
Deferred									183	93	(24)
Total income tax expense									920	864	698
Aflac U.S.
Reconciliation of Provision of Income Taxes [Line Items]
Current									69	608	(91)
Deferred									152	(409)	(1,193)
Total income tax expense									$ 221	$ 199	$ (1,284)