INCOME TAXES - Components of Income Tax Expense (Benefit) Applicable to Pretax Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Provision of Income Taxes [Line Items]
Current									$ 1,379	$ 631	$ 884
Deferred									(316)	(1,217)	524
Total income tax expense	$ 197	$ 301	$ 299	$ 265	$ (1,585)	$ 359	$ 332	$ 306	1,063	(586)	1,408
Aflac U.S.
Reconciliation of Provision of Income Taxes [Line Items]
Current									608	(91)	234
Deferred									(409)	(1,193)	388
Total income tax expense									199	(1,284)	622
Aflac Japan
Reconciliation of Provision of Income Taxes [Line Items]
Current									771	722	650
Deferred									93	(24)	136
Total income tax expense									$ 864	$ 698	$ 786