POLICY LIABILITIES - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Future Policy Benefit, by Product Segment [Line Items]
Percentage of future policy benefits out of policy liabilities	85.00%
Percentage of unpaid policy claims out of policy liabilities	5.00%
Percentage of unearned premiums out of policy liabilities	3.00%
Percentage of other poliyholders' funds out of policy liabilities	7.00%
Percentage of advanced premiums to unearned premiums	60.00%	64.00%
Percentage of annuities to other policyholders' fund	97.00%	97.00%
Aflac Japan
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits, interest rate	3.10%	3.20%	3.30%
Aflac U.S.
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for future policy benefits, interest rate	5.20%	5.30%	5.30%
Health insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year claims and claims adjustment expense	$ (540)	$ (552)	$ (563)
Health insurance | Aflac Japan
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year claims and claims adjustment expense	$ 334
Percentage of total prior year claims and claims adjustment expense	62.00%
Liability for unpaid claims and claims adjustment expense, foreign currency translation gain (loss)	$ 7
Prior year claims and claims adjustment expense excluding effect of foreign currency	$ 327
Percentage of total prior year claims and claims adjustment expense excluding effect of foreign currency	61.00%
Health insurance | Aflac U.S.
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year claims and claims adjustment expense	$ 206
Percentage of total prior year claims and claims adjustment expense	38.00%