BENEFIT PLANS - Fair Value Hierarchy Levels of Funded Pension Plans' Assets (Detail) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Japan
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 270	$ 229	$ 198
U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	448	359	$ 336
U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	448	359
Japanese equity securities | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	37	28
International equity securities | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	50	40
Japanese bonds | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	91	79
International bonds | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	62	55
Insurance contracts | Japan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	30	27
Cash and cash equivalents | Japan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	0
Cash and cash equivalents | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	1	1
U.S. large cap equity securities | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	124	104
U.S. mid cap equity securities | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	22	19
Real estate equity funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	13	10
International equity fund | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	108	85
Fixed income bond funds | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	175	136
Aflac Incorporated common stock | U.S. | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 5	$ 4