BENEFIT PLANS - Fair Value Hierarchy Levels of Funded Pension Plans' Assets (Detail) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Japan
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 344	$ 289	$ 270
U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	644	465	$ 448
Japanese equity securities | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17	14
International equity securities | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	67	50
Japanese bonds | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	20	34
International bonds | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	207	160
Insurance contracts | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	33	31
U.S. large cap equity securities | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	179	120
U.S. mid cap equity securities | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	22	17
Real estate equity funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	16	13
International equity funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	112	92
Fixed income bond funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	209	179
Aflac Incorporated common stock | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	6	5
Cash and cash equivalents | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 100	$ 39