BUSINESS SEGMENT AND FOREIGN INFORMATION - Operations by Segment - Pretax Earnings (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, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[1]									$ 4,416	$ 4,461	$ 4,354
Net investment gains (losses)	[2],[3],[4],[5],[6]									(229)	(15)	(297)
Other income (loss)										(28)	(1)	(74)
Earnings before income taxes		$ 1,216	$ 1,153	$ 1,070	$ 720	$ 1,058	$ 1,036	$ 1,109	$ 1,242	4,159	4,445	3,983
Income taxes applicable to pretax adjusted earnings										864	1,147	1,129
Effect of foreign currency translation on after tax adjusted earnings										31	15	28
Interest expense on debt										167	135	122
Aflac Japan
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[3],[6]									3,263	3,261	3,208
Hedge costs										206	257	236
Net interest cash flows from derivatives										9	(17)
Aflac U.S.
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[5]									1,268	1,272	1,285
Net interest cash flows from derivatives										3
Corporate and other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[2],[4]									(115)	(72)	(139)
Hedge income										97	89	36
Gain (loss) on change in fair value of derivative, interest rate component										$ 56	$ 66	$ 67

[1] Includes $167, $135 and $122 of interest expense on debt in 2020, 2019 and 2018, respectively.

[2] A gain of $56, $66 and $67 in 2020, 2019 and 2018, respectively, related to the interest rate component of the change in fair value of foreign currency swaps on notes payable have been reclassified from net investment gains (losses) and included in adjusted earnings when analyzing operations.

[3] 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.

[4] 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.

[5] 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.

[6] 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.