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Schedule II - Aflac Incorporated (Parent Only) - Additional Information (Detail)
¥ in Millions, $ in Millions
 1 Months Ended
Apr. 30, 2020
USD ($)
Jan. 31, 2020
USD ($)
Sep. 30, 2019
JPY (¥)
Feb. 29, 2012
USD ($)
Dec. 31, 2020
JPY (¥)
Mar. 31, 2020
JPY (¥)
series
Dec. 31, 2019
JPY (¥)
series
Oct. 31, 2018
JPY (¥)
series
Sep. 30, 2016
USD ($)
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 4 3 2
Senior Notes                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount           ¥ 57,000 ¥ 38,000 ¥ 53,400 $ 700
.500% senior notes due December 2029                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 12,600   ¥ 12,600    
Debt instrument, interest rate         0.50%   0.50%    
.843% senior notes due December 2031                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 9,300   ¥ 9,300    
Debt instrument, interest rate         0.843%   0.843%    
.934% senior notes due December 2034                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 9,800   ¥ 9,800    
Debt instrument, interest rate         0.934%   0.934%    
1.122% senior notes due December 2039                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 6,300   ¥ 6,300    
Debt instrument, interest rate         1.122%   1.122%    
Yen-denominated loans                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount     ¥ 30,000            
Yen-denominated loan variable interest rate due September 2026                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount     ¥ 5,000   ¥ 5,000   ¥ 5,000    
Debt instrument, interest rate         0.43%   0.42%    
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     ¥ 25,000   ¥ 25,000   ¥ 25,000    
Debt instrument, interest rate         0.58%   0.57%    
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            
3.60% senior notes due April 2030                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount | $ $ 1,000                
Debt instrument, interest rate 3.60%       3.60%        
Debt instrument, redemption, description These notes are redeemable at the Parent Company's option in whole at any time or in part from time to time at a redemption price equal to the greater of: (i) the aggregate principal amount of the notes to be redeemed or (ii) the amount equal to the sum of the present values of the remaining scheduled payments for principal of and interest on the notes to be redeemed, not including any portion of the payments of interest accrued as of such redemption date, discounted to such redemption date on a semiannual basis at the yield to maturity for a U.S. Treasury security with a maturity comparable to the remaining term of the notes, plus 45 basis points, plus in each case, accrued and unpaid interest on the principal amount of the notes to be redeemed to, but excluding, such redemption date.                
.300% senior notes due September 2025                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 12,400 ¥ 12,400      
Debt instrument, interest rate         0.30% 0.30%      
.550% senior notes due March 2030                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 13,300 ¥ 13,300      
Debt instrument, interest rate         0.55% 0.55%      
.750% senior notes due March 2032                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 20,700 ¥ 20,700      
Debt instrument, interest rate         0.75% 0.75%      
.830% senior notes due March 2035                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 10,600 ¥ 10,600      
Debt instrument, interest rate         0.83% 0.83%      
4.00% senior notes paid January 2020                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount | $       $ 350          
Debt instrument, interest rate   4.00%   4.00%     4.00%    
Debt instrument, redemption, description       These notes are redeemable at the Parent Company's option in whole at any time or in part from time to time at a redemption price equal to the greater of: (i) the principal amount of the notes or (ii) the present value of the remaining scheduled payments of principal and interest to be redeemed, discounted to the redemption date, plus accrued and unpaid interest.          
Repayments of debt | $   $ 350              
Parent Company | Senior Notes                  
Condensed Financial Statements, Captions [Line Items]                  
Number of series of senior notes issued through a U.S. public debt offering (in series) | series           4      
Debt instrument, principal amount           ¥ 57,000      
Parent Company | .500% senior notes due December 2029                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 12,600   ¥ 12,600    
Debt instrument, interest rate         0.50%   0.50%    
Parent Company | .843% senior notes due December 2031                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 9,300   ¥ 9,300    
Debt instrument, interest rate         0.843%   0.843%    
Parent Company | .934% senior notes due December 2034                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 9,800   ¥ 9,800    
Debt instrument, interest rate         0.934%   0.934%    
Parent Company | 1.122% senior notes due December 2039                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 6,300   ¥ 6,300    
Debt instrument, interest rate         1.122%   1.122%    
Parent Company | Yen-denominated loan variable interest rate due September 2026                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 5,000   ¥ 5,000    
Debt instrument, interest rate         0.43%   0.42%    
Parent Company | Yen-denominated loan variable interest rate due September 2029                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 25,000   ¥ 25,000    
Debt instrument, interest rate         0.58%   0.57%    
Parent Company | 3.60% senior notes due April 2030                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount | $ $ 1,000                
Debt instrument, interest rate 3.60%       3.60%        
Debt instrument, redemption, description These notes are redeemable at the Parent Company's option in whole at any time or in part from time to time at a redemption price equal to the greater of: (i) the aggregate principal amount of the notes to be redeemed or (ii) the amount equal to the sum of the present values of the remaining scheduled payments for principal of and interest on the notes to be redeemed, not including any portion of the payments of interest accrued as of such redemption date, discounted to such redemption date on a semiannual basis at the yield to maturity for a U.S. Treasury security with a maturity comparable to the remaining term of the notes, plus 45 basis points, plus in each case, accrued and unpaid interest on the principal amount of the notes to be redeemed to, but excluding, such redemption date.                
Parent Company | .300% senior notes due September 2025                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 12,400 ¥ 12,400      
Debt instrument, interest rate         0.30% 0.30%      
Parent Company | .550% senior notes due March 2030                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 13,300 ¥ 13,300      
Debt instrument, interest rate         0.55% 0.55%      
Parent Company | .750% senior notes due March 2032                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 20,700 ¥ 20,700      
Debt instrument, interest rate         0.75% 0.75%      
Parent Company | .830% senior notes due March 2035                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, principal amount         ¥ 10,600 ¥ 10,600      
Debt instrument, interest rate         0.83% 0.83%      
Parent Company | 4.00% senior notes paid January 2020                  
Condensed Financial Statements, Captions [Line Items]                  
Debt instrument, interest rate   4.00%         4.00%    
Repayments of debt | $   $ 350              
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%            
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%