POLICY LIABILITIES - Summary of Discounted Expected Future Gross Premiums and Future Benefits and Expenses (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	$ 114,691	$ 121,753
Benefits and Expenses	123,453	132,131
Cancer | Aflac Japan
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	44,795	48,363
Benefits and Expenses	46,389	50,161	$ 54,766
Medical and other health | Aflac Japan
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	28,312	30,757
Benefits and Expenses	23,331	25,257	27,419
Life insurance | Aflac Japan
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	10,274	11,240
Benefits and Expenses	27,249	29,731	31,954
Life insurance | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	1,963	1,914
Benefits and Expenses	1,775	1,764	1,526
Accident | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	6,293	6,369
Benefits and Expenses	3,070	3,109	3,098
Disability | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	4,476	4,488
Benefits and Expenses	2,423	2,422	2,445
Critical Care | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	12,295	12,417
Benefits and Expenses	11,102	11,290	11,489
Hospital Indemnity | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	3,425	3,419
Benefits and Expenses	1,938	1,943	2,074
Dental/Vision | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	794	807
Benefits and Expenses	474	478	488
Other | Aflac Japan
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	1,391	1,512
Benefits and Expenses	4,772	5,178	5,582
Other | Aflac U.S.
Liability for Future Policy Benefit, Activity [Line Items]
Gross Premiums	673	467
Benefits and Expenses	$ 930	$ 798	$ 622