INCOME TAXES - Components of Income Tax Expense (Benefit) Applicable to Pretax Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Provision of Income Taxes [Line Items]
Current									$ 631	$ 884	$ 1,288
Deferred									(1,217)	524	41
Total income tax expense	$ (1,585)	$ 359	$ 332	$ 306	$ 402	$ 334	$ 286	$ 386	(586)	1,408	1,329
Aflac U.S.
Reconciliation of Provision of Income Taxes [Line Items]
Current									(91)	234	225
Deferred									(1,193)	388	(1)
Total income tax expense									(1,284)	622	224
Aflac Japan
Reconciliation of Provision of Income Taxes [Line Items]
Current									722	650	1,063
Deferred									(24)	136	42
Total income tax expense									$ 698	$ 786	$ 1,105