SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Summary of Signficant Accounting Policies - Additional Information (Detail) $ in Millions		12 Months Ended
	Feb. 13, 2018	Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)		Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Stock split, conversion ratio	2
Common stock as a percentage of two-for-one stock split dividend	100.00%
Aflac Japan
Significant Accounting Policies [Line Items]
Percentage of the Company assets		84.00%		83.00%
Aflac Japan's percentage of the Company's total revenues		70.00%		70.00%		71.00%
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
Accumulated other comprehensive income (loss) | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes		$ (148)	[1]	$ 0	[1]	$ 0	[1]		$ (148)
Accumulated other comprehensive income (loss) | Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes		374	[1]	0	[1]	0	[1]		374
Retained earnings | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes		148	[1]	0	[1]	0	[1]		148
Retained earnings | Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes		$ (374)	[1]	$ 0	[1]	$ 0	[1]		$ (374)
Scenario, Forecast | Lower Limit | Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on assets								$ 100
New accounting pronouncement or change in accounting principle, cumulative effect of change on liabilities								100
Scenario, Forecast | Upper Limit | Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on assets								200
New accounting pronouncement or change in accounting principle, cumulative effect of change on liabilities								$ 200

[1] See Note 1 of the Notes to the Consolidated Financial Statements for the adoption of accounting guidance on January 1, 2018.