BUSINESS SEGMENT AND FOREIGN INFORMATION - Operations by Segment - Revenues (Details) - 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
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance		$ 4,653	$ 4,623	$ 4,664	$ 4,681	$ 4,671	$ 4,736	$ 4,681	$ 4,691	$ 18,622	$ 18,780	$ 18,677
Total revenues		$ 5,913	$ 5,665	$ 5,407	$ 5,162	$ 5,603	$ 5,536	$ 5,511	$ 5,657	22,147	22,307	21,758
Total adjusted revenues										22,320	22,256	21,988
Net investment gains (losses)	[1],[2],[3],[4]									(173)	51	(230)
Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,253	3,445	3,674
Aflac Japan
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										12,670	12,772	12,762
Adjusted net investment income	[1],[4]									2,659	2,496	2,403
Other income (loss)										42	45	41
Total revenues										15,371	15,313	15,206
Hedge costs										206	257	236
Net interest cash flows from derivatives										9	(17)
Aflac Japan | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										6,119	6,031	5,849
Aflac Japan | Medical and other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,596	3,582	3,516
Aflac Japan | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										2,955	3,159	3,397
Aflac U.S.
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										5,758	5,808	5,708
Adjusted net investment income	[3]									705	720	727
Other income (loss)										102	22	8
Total revenues										6,565	6,550	6,443
Net interest cash flows from derivatives										3
Aflac U.S. | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										1,275	1,309	1,311
Aflac U.S. | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										298	286	278
Aflac U.S. | Accident and disability
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										2,614	2,665	2,611
Aflac U.S. | Other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										1,571	1,548	1,508
Corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	[2]									384	393	339
Hedge income										$ 97	$ 89	$ 36

[1] Amortized hedge costs of $206, $257 and $236 in 2020, 2019 and 2018, respectively, related to certain foreign currency exposure management strategies have been reclassified from net investment gains (losses) and reported as a deduction from net investment income when analyzing operations.

[2] Amortized hedge income of $97, $89 and $36 in 2020, 2019 and 2018, respectively, related to certain foreign currency exposure management strategies has been reclassified from net 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 $3 in 2020 have been reclassified from net investment gains (losses) and included in adjusted earnings as a component of net investment income.

[4] Net interest cash flows from derivatives associated with certain investment strategies of $9 and $(17) in 2020 and 2019, respectively, and an immaterial amount in 2018, have been reclassified from net investment gains (losses) and included in adjusted earnings as a component of net investment income.