BUSINESS SEGMENT AND FOREIGN INFORMATION - Operations by Segment - Pretax Earnings (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, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[1]									$ 4,461	$ 4,354	$ 4,087
Realized investment gains (losses)	[2],[3],[4],[5]									(15)	(297)	0
Other income (loss)	[6]									(1)	(74)	(69)
Earnings before income taxes		$ 1,058	$ 1,036	$ 1,109	$ 1,242	$ 722	$ 1,146	$ 1,131	$ 982	4,445	3,983	4,018
Income taxes applicable to pretax adjusted earnings										1,147	1,129	1,370
Effect of foreign currency translation on after tax adjusted earnings										15	28	(41)
Interest expense on debt										135	122	122
Aflac Japan
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[3],[5]									3,261	3,208	3,054
Hedge costs										257	236	228
Net interest cash flows from derivatives										(17)
Aflac U.S.
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings										1,272	1,285	1,245
Corporate and other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pretax adjusted earnings	[2],[4]									(72)	(139)	(212)
Hedge income										89	36
Gain (loss) on change in fair value of derivative, interest rate component										$ 66	$ 67	77
5.50% subordinated notes due September 2052
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Expense on extinguishment of debt												$ 13

[1] Includes $135, $122 and $122 of interest expense on debt in 2019, 2018 and 2017

[2] A gain of $66 in 2019, $67 in 2018 and $77 in 2017, related to the interest rate component of the change in fair value of foreign currency swaps on notes payable have been reclassified from realized investment gains (losses) and included in adjusted earnings when analyzing operations.

[3] 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.

[4] 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.

[5] 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.

[6] Includes expense of $13 in 2017 for the early extinguishment of debt