BENEFIT PLANS - Fair Value Hierarchy Levels of Funded Pension Plans' Assets (Detail) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Japan
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 289	$ 270	$ 229
U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	465	448	$ 359
Japanese equity securities | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	14	37
International equity securities | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	50	50
Japanese bonds | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	34	91
International bonds | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	160	62
Insurance contracts | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	31	30
U.S. large cap equity securities | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	120	124
U.S. mid cap equity securities | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	17	22
Real estate equity funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	13	13
International equity funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	92	108
Fixed income bond funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	179	175
Aflac Incorporated common stock | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	5	5
Cash and cash equivalents | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 39	$ 1