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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Summary of Signficant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2020 | Jan. 01, 2019 | Jan. 01, 2018
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of change in accounting principle, net of income taxes |  |  | $ 0 |  |  |  |
Payments to Acquire Businesses, Gross | $ 75 |  |  |  |  |  |
Maximum |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Business Combination, Contingent Consideration, Liability | $ 21 | $ 21 |  |  |  |  |
Aflac Japan |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Percentage of the Company's total revenues |  | 69.00% | 70.00% | 70.00% |  |  |
Percentage of the Company's total assets | 83.00% | 83.00% | 84.00% |  |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
New accounting pronouncement or change in accounting principle, cumulative effect of change on assets |  |  |  |  |  | $ 134 |
New accounting pronouncement or change in accounting principle, cumulative effect of change on liabilities |  |  |  |  |  | $ 134 |
Accounting Standards Update 2019-04 | Forecast |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
New accounting pronouncement or change in accounting principle effect of adoption reclassification from held to maturity to available for sale debt securities |  |  |  |  | $ 6,900 |  |
New accounting pronouncement or change in accounting principle effect of adoption unrealized gain loss net of tax reclassification to accumulated other comprehensive income |  |  |  |  | 800 |  |
Accumulated other comprehensive income (loss) |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of change in accounting principle, net of income taxes |  |  | $ 226 |  |  |  |
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)
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
Retained earnings |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of change in accounting principle, net of income taxes |  |  | $ (226) |  |  |  |
Retained earnings | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of change in accounting principle, net of income taxes |  |  |  |  |  |  | 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)
Retained earnings | Accounting Standards Update 2019-04 | Forecast |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of change in accounting principle, net of income taxes |  |  |  |  | $ (56) |  |

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