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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2021 USD ($) segment | Sep. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) | Jan. 01, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of reportable insurance business segments | segment | 2 |  |  |  |  |  |
Accumulated other comprehensive income (loss) | $ 7,662 | $ 7,917 | $ 8,019 |  | $ 8,934 | $ 6,750 | $ 6,615
Lower Limit | Pro Forma | Accounting Standards Update 2018-12 | Cumulative effect, period of adoption, adjustment |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Accumulated other comprehensive income (loss) |  |  |  | $ (18,000) |  |  |
Upper Limit | Pro Forma | Accounting Standards Update 2018-12 | Cumulative effect, period of adoption, adjustment |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Accumulated other comprehensive income (loss) |  |  |  | $ (20,000) |  |  |
Aflac Japan |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Percentage of the Company's total revenues | 69.00% | 68.00% |  |  |  |  |
Percentage of the Company's total assets | 82.00% |  |  |  | 83.00% |  |

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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)(a)(4)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(23)(a)(3)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669686-108580 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (g)(2)(ii) -URI http://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641 Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e681-108580 + Details
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