BUSINESS SEGMENT AND FOREIGN INFORMATION - Operations by Segment - Revenues (Details) - 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
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance		$ 4,671	$ 4,736	$ 4,681	$ 4,691	$ 4,591	$ 4,636	$ 4,706	$ 4,745	$ 18,780	$ 18,677	$ 18,531
Net investment income		886	936	878	878	874	870	862	837	3,578	3,442	3,220
Total revenues		$ 5,603	$ 5,536	$ 5,511	$ 5,657	$ 5,126	$ 5,577	$ 5,589	$ 5,464	22,307	21,758	21,667
Total adjusted revenues										22,256	21,988	21,589
Realized investment gains (losses)	[1],[2],[3]									51	(230)	78
Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,445	3,674	4,034
Aflac Japan
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										12,772	12,762	12,752
Net investment income, less amortized hedge costs	[1],[3]									2,496	2,403	2,235
Other income (loss)										45	41	41
Net investment income										2,753	2,639	2,463
Total revenues										15,313	15,206	15,028
Hedge costs										257	236	228
Net interest cash flows from derivatives										(17)
Aflac Japan | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										6,031	5,849	5,612
Aflac Japan | Medical and other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,582	3,516	3,379
Aflac Japan | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,159	3,397	3,761
Aflac U.S.
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										5,808	5,708	5,563
Other income (loss)										22	8	5
Net investment income										720	727	721
Total revenues										6,550	6,443	6,289
Aflac U.S. | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										1,309	1,311	1,308
Aflac U.S. | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										286	278	273
Aflac U.S. | Accident and disability
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										2,665	2,611	2,537
Aflac U.S. | Other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										1,548	1,508	1,445
Corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	[2]									393	339	$ 272
Hedge income										$ 89	$ 36

[1] Amortized hedge costs of $257, $236 and $228 in 2019, 2018 and 2017, respectively, related to certain foreign currency exposure management strategies have been reclassified from realized investment gains (losses) and reported as a deduction from net investment income when analyzing operations.

[2] Amortized hedge income of $89 in 2019 and $36 in 2018 related to certain foreign currency exposure management strategies has been reclassified from realized investment gains (losses) and reported as an increase to net investment income when analyzing operations.

[3] Net interest cash flows from derivatives associated with certain investment strategies of $(17) in 2019 and an immaterial amount in 2018 have been reclassified from realized investment gains (losses) and included in adjusted earnings as a component of net investment income.