POLICY LIABILITIES - Additional Information (Details) - Health insurance - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years claims and claims adjustment expense	$ (192)	$ (111)
Aflac Japan
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years claims and claims adjustment expense	$ (97)
Percentage of total prior year claims and claims adjustment expense	51.00%
Liability for unpaid claims and claims adjustment expense, foreign currency translation gain (loss)	$ (5)
Prior year claims and claims adjustment expense excluding effect of foreign currency	$ (92)
Percentage of total prior year claims and claims adjustment expense excluding effect of foreign currency	48.00%