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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 |

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