SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Summary of Signficant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2018
Aflac Japan
Significant Accounting Policies [Line Items]
Percentage of the Company assets	83.00%	83.00%
Aflac Japan's percentage of the Company's total revenues	70.00%	71.00%	70.00%
Accounting Standards Update 2015-03
Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Liabilities			$ 40
Upper Limit
Significant Accounting Policies [Line Items]
Estimation of Percent of Leases Within Scope of the Guidance to Total Assets	1.00%
Estimation of Percent of Revenues Within Scope of the Guidance to Total Revenues	1.00%
Scenario, Forecast | Accumulated other comprehensive income (loss) | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ (133)
Scenario, Forecast | Retained earnings | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 133