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Schedule II - Aflac Incorporated (Parent Only) - Additional Information (Detail) $ in Millions, ¥ in Billions | 1 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 JPY (¥) | Dec. 31, 2019 JPY (¥) series | Dec. 31, 2018 JPY (¥) | Oct. 31, 2018 JPY (¥) series | Sep. 30, 2016 USD ($) series
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Number of series of senior notes issued through a U.S. public debt offering (in series) | series |  | 4 |  | 3 | 2
Senior Notes |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 38.0 |  | ¥ 53.4 | $ 700
.500% senior notes due December 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 12.6 |  |  |
Debt instrument, interest rate |  | 0.50% |  |  |
.843% senior notes due December 2031 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 9.3 |  |  |
Debt instrument, interest rate |  | 0.843% |  |  |
.934% senior notes due December 2034 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 9.8 |  |  |
Debt instrument, interest rate |  | 0.934% |  |  |
1.122% senior notes due December 2039 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 6.3 |  |  |
Debt instrument, interest rate |  | 1.122% |  |  |
Yen-denominated loans |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount | ¥ 30.0 |  |  |  |
Yen-denominated loan variable interest rate due September 2026 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount | ¥ 5.0 | ¥ 5.0 | ¥ 5.0 |  |
Debt instrument, interest rate |  | 0.42% | 0.32% |  |
Debt instrument, description of variable rate basis | bears interest at a rate per annum equal to the Tokyo interbank market rate (TIBOR), or alternate TIBOR, if applicable, plus the applicable TIBOR margin |  |  |  |
Yen-denominated loan variable interest rate due September 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount | ¥ 25.0 | ¥ 25.0 | ¥ 25.0 |  |
Debt instrument, interest rate |  | 0.57% | 0.47% |  |
Debt instrument, description of variable rate basis | bears interest at a rate per annum equal to the TIBOR, or alternate TIBOR, if applicable, plus the applicable TIBOR margin |  |  |  |
Parent Company |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Number of series of senior notes issued through a U.S. public debt offering (in series) | series |  | 4 |  |  |
Parent Company | Senior Notes |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 38.0 |  |  |
Parent Company | .500% senior notes due December 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 12.6 |  |  |
Debt instrument, interest rate |  | 0.50% |  |  |
Parent Company | .843% senior notes due December 2031 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 9.3 |  |  |
Debt instrument, interest rate |  | 0.843% |  |  |
Parent Company | .934% senior notes due December 2034 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 9.8 |  |  |
Debt instrument, interest rate |  | 0.934% |  |  |
Parent Company | 1.122% senior notes due December 2039 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount |  | ¥ 6.3 |  |  |
Debt instrument, interest rate |  | 1.122% |  |  |
Parent Company | Yen-denominated loans |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount | ¥ 30.0 |  |  |  |
Parent Company | Yen-denominated loan variable interest rate due September 2026 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount | ¥ 5.0 | ¥ 5.0 | ¥ 5.0 |  |
Debt instrument, interest rate |  | 0.42% | 0.32% |  |
Debt instrument, description of variable rate basis | bears interest at a rate per annum equal to the Tokyo interbank market rate (TIBOR), or alternate TIBOR, if applicable, plus the applicable TIBOR margin |  |  |  |
Parent Company | Yen-denominated loan variable interest rate due September 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, principal amount | ¥ 25.0 | ¥ 25.0 | ¥ 25.0 |  |
Debt instrument, interest rate |  | 0.57% | 0.47% |  |
Debt instrument, description of variable rate basis | bears interest at a rate per annum equal to the TIBOR, or alternate TIBOR, if applicable, plus the applicable TIBOR margin |  |  |  |
Lower Limit | Yen-denominated loan variable interest rate due September 2026 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 0.30% |  |  |  |
Lower Limit | Yen-denominated loan variable interest rate due September 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 0.45% |  |  |  |
Lower Limit | Parent Company | Yen-denominated loan variable interest rate due September 2026 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 0.30% |  |  |  |
Lower Limit | Parent Company | Yen-denominated loan variable interest rate due September 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 0.45% |  |  |  |
Upper Limit | Yen-denominated loan variable interest rate due September 2026 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 0.70% |  |  |  |
Upper Limit | Yen-denominated loan variable interest rate due September 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 1.00% |  |  |  |
Upper Limit | Parent Company | Yen-denominated loan variable interest rate due September 2026 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 0.70% |  |  |  |
Upper Limit | Parent Company | Yen-denominated loan variable interest rate due September 2029 |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, basis spread on variable rate | 1.00% |  |  |  |

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