BUSINESS SEGMENT AND FOREIGN INFORMATION - Operations by Segment - Revenues (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance		$ 4,591	$ 4,636	$ 4,706	$ 4,745	$ 4,580	$ 4,648	$ 4,665	$ 4,638	$ 18,677	$ 18,531	$ 19,225
Net investment income		874	870	862	837	812	811	802	794	3,442	3,220	3,278
Total revenues		$ 5,126	$ 5,577	$ 5,589	$ 5,464	$ 5,424	$ 5,506	$ 5,428	$ 5,309	21,758	21,667	22,559
Total adjusted revenues										21,988	21,589	22,387
Realized investment gains (losses)	[1],[2],[3]									(230)	78	172
Gain (loss) on change in fair value of derivative, corporate										36
Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,674	4,034	4,740
Aflac Japan
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										12,762	12,752	13,537
Net investment income, less amortized hedge costs										2,403	2,235	2,368
Other income (loss)										41	41	40
Net investment income										2,639	2,463	2,554
Total revenues										15,206	15,028	15,945
Hedge cost										236	228	186
Aflac Japan | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										5,849	5,612	5,639
Aflac Japan | Medical and other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,516	3,379	3,429
Aflac Japan | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										3,397	3,761	4,469
Aflac U.S.
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										5,708	5,563	5,454
Other income (loss)										8	5	10
Net investment income										727	721	703
Total revenues										6,443	6,289	6,167
Aflac U.S. | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										1,311	1,308	1,299
Aflac U.S. | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										278	273	271
Aflac U.S. | Accident and disability
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										2,611	2,537	2,469
Aflac U.S. | Other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums, principally supplemental health insurance										1,508	1,445	1,415
Corporate and other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues										$ 339	$ 272	$ 275

[1] Amortized hedge costs in Aflac Japan were partially offset by derivatives entered into as part of corporate activities and resulted in a benefit of $36 for 2018, which has been reclassified from realized investment gains (losses) and reported as an increase in net investment income when analyzing operations.

[2] Amortized hedge costs related to hedging U.S. dollar-denominated investments held in Aflac Japan were $236, $228 and $186 for 2018, 2017 and 2016, respectively, and have been reclassified from realized investment gains (losses) and reported as a deduction from net investment income when analyzing segment operations.

[3] An immaterial amount of net interest cash flows from derivatives associated with certain investment strategies in 2018, were reclassified from realized investment gains (losses) into net investment income when analyzing operations.