POLICY LIABILITIES - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Future Policy Benefit, by Product Segment [Line Items]
Percentage of future policy benefits out of policy liabilities	83.00%
Percentage of unpaid policy claims out of policy liabilities	4.00%
Percentage of unearned premiums out of policy liabilities	6.00%
Percentage of other poliyholders' funds out of policy liabilities	7.00%
Percentage of advanced premiums to unearned premiums	73.00%	76.00%
Percentage of annuities to other policyholders' fund	98.00%	98.00%
Aflac Japan
Liability for Future Policy Benefit, by Product Segment [Line Items]
Weighted-average interest rates reflected in the consolidated statements of earnings for future policy benefits	3.40%	3.50%	3.60%
Aflac U.S.
Liability for Future Policy Benefit, by Product Segment [Line Items]
Weighted-average interest rates reflected in the consolidated statements of earnings for future policy benefits	5.40%	5.50%	5.60%
Health insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year claims and claims adjustment expense	$ 518	$ 465	$ 353
Health insurance | Aflac Japan
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year claims and claims adjustment expense	$ 383
Percentage of total prior year claims and claims adjustment expense	74.00%
Liability for unpaid claims and claims adjustment expense, foreign currency translation gain (loss)	$ 18
Prior year claims and claims adjustment expense excluding effect of foreign currency	$ 400
Percentage of total prior year claims and claims adjustment expense excluding effect of foreign currency	77.00%