BUSINESS SEGMENT AND FOREIGN INFORMATION - Operations by Segment - Revenues (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net		$ 4,580	$ 4,648	$ 4,665	$ 4,638	$ 4,778	$ 5,022	$ 4,823	$ 4,602	$ 18,531	$ 19,225	$ 17,570
Net investment income		812	811	802	794	813	842	822	801	3,220	3,278	3,135
Total revenues		$ 5,424	$ 5,506	$ 5,428	$ 5,309	$ 5,955	$ 5,716	$ 5,437	$ 5,451	21,667	22,559	20,872
Total Operating Revenues										21,667	22,472	20,778
Realized investment gains (losses), non-operating	[1],[2],[3],[4]									0	87	94
Gain (loss) on change in fair value of derivative, interest rate component, operating										77	85	85
Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										4,034	4,740	4,289
Aflac Japan
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										12,752	13,537	12,046
Net investment income, less amortized hedge costs	[1]									2,235	2,368	2,364
Net investment income										2,463	2,554	2,436
Other income (loss)										41	40	31
Total revenues										15,028	15,945	14,441
Hedge cost										(228)	(186)	(72)
Aflac Japan | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										5,612	5,639	4,933
Aflac Japan | Medical and other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										3,379	3,429	3,092
Aflac Japan | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										3,761	4,469	4,021
Aflac U.S.
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										5,563	5,454	5,347
Net investment income										721	703	678
Other income (loss)										5	10	8
Total revenues										6,289	6,167	6,033
Aflac U.S. | Cancer
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										1,308	1,299	1,293
Aflac U.S. | Life insurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										273	271	268
Aflac U.S. | Accident and disability
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										2,537	2,469	2,391
Aflac U.S. | Other health
Segment Reporting, Revenue Reconciling Item [Line Items]
Premiums Earned, Net										1,445	1,415	1,395
Other business segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues										140	275	225
Total Business Segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues										21,457	22,387	20,699
Corporate and eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues										$ 210	$ 85	$ 79

[1] Amortized hedge costs related to hedging U.S. dollar-denominated investments held in Aflac Japan were $228, $186 and $72 for 2017, 2016 and 2015 respectively, and have been reclassified from realized investment gains (losses) and reported as a deduction from pretax operating earnings when analyzing segment operations to conform to current year reporting.

[2] Excluding a gain of $77 in 2017 and $85 in both 2016 and 2015 related to the interest rate component of the change in fair value of foreign currency swaps on notes payable which is classified as an operating gain when analyzing segment operations

[3] Prior year foreign currency transaction gains and losses have been reclassified from other non-operating income (loss) to realized investment gains (losses) to conform to current-year reporting classifications. These reclassifications had no impact on total earnings before income taxes.

[4] Prior year foreign currency transaction gains and losses have been reclassified from other non-operating income (loss) to realized investment gains (losses) to conform to current-year reporting classifications. These reclassifications had no impact on total revenues.