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Schedule II - Aflac Incorporated (Parent Only) - Additional Information (Detail)
$ in Millions, ¥ in Billions
1 Months Ended
Nov. 30, 2017
USD ($)
Oct. 31, 2017
JPY (¥)
Feb. 28, 2017
USD ($)
Jan. 31, 2017
JPY (¥)
Sep. 30, 2012
USD ($)
Jul. 31, 2012
USD ($)
Feb. 29, 2012
USD ($)
Dec. 31, 2017
JPY (¥)
Dec. 31, 2016
Oct. 31, 2012
USD ($)
2.108% subordinated notes due October 2047                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Face Amount | ¥   ¥ 60.0           ¥ 60.0    
Debt Instrument, Interest Rate, Stated Percentage   2.108%           2.108%    
Debt instrument, term   30 years                
Debt Instrument, Redemption, Description   The debentures are redeemable (i) at any time, in whole but not in part, upon the occurrence of certain tax events or certain rating agency events, as specified in the indenture governing the terms of the debentures or (ii) on or after October 23, 2027, in whole or in part, at a redemption price equal to their principal amount plus accrued and unpaid interest to, but excluding, the date of redemption.                
Debt Instrument, Call Date, Earliest   Oct. 23, 2027                
Debt Instrument, Interest Rate Terms   The debentures bear interest at an initial rate of 2.108% per annum through October 22, 2027, or earlier redemption. Thereafter, the rate of the interest of the debentures will be reset every five years at a rate of interest equal to the then-current JPY 5-year Swap Offered Rate plus 205 basis points.                
.932% senior notes due January 2027                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Face Amount | ¥       ¥ 60.0       ¥ 60.0    
Debt Instrument, Interest Rate, Stated Percentage       0.932%       0.932%    
Debt instrument, term       10 years            
2.65% senior notes paid February 2017                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Face Amount           $ 250 $ 400      
Debt Instrument, Interest Rate, Stated Percentage     2.65%     2.65% 2.65%   2.65%  
Debt instrument, term           5 years 5 years      
Repayments of debt     $ 650              
5.50% subordinated notes due September 2052                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Face Amount         $ 450         $ 50
Debt Instrument, Interest Rate, Stated Percentage 5.50%       5.50%       5.50% 5.50%
Debt instrument, term         40 years          
Repayments of debt $ 500                  
Parent Company | 2.108% subordinated notes due October 2047                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Face Amount | ¥   ¥ 60.0           ¥ 60.0    
Debt Instrument, Interest Rate, Stated Percentage   2.108%           2.108%    
Debt instrument, term   30 years                
Debt Instrument, Redemption, Description   The debentures are redeemable (i) at any time, in whole but not in part, upon the occurrence of certain tax events or certain rating agency events, as specified in the indenture governing the terms of the debentures or (ii) on or after October 23, 2027, in whole or in part, at a redemption price equal to their principal amount plus accrued and unpaid interest to, but excluding, the date of redemption.                
Debt Instrument, Call Date, Earliest   Oct. 23, 2027                
Debt Instrument, Interest Rate Terms   The debentures bear interest at an initial rate of 2.108% per annum through October 22, 2027, or earlier redemption. Thereafter, the rate of the interest of the debentures will be reset every five years at a rate of interest equal to the then-current JPY 5-year Swap Offered Rate plus 205 basis points.                
Parent Company | .932% senior notes due January 2027                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Face Amount | ¥       ¥ 60.0       ¥ 60.0    
Debt Instrument, Interest Rate, Stated Percentage       0.932%       0.932%    
Debt instrument, term       10 years            
Parent Company | 2.65% senior notes paid February 2017                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Interest Rate, Stated Percentage     2.65%           2.65%  
Repayments of debt     $ 650              
Parent Company | 5.50% subordinated notes due September 2052                    
Condensed Financial Statements, Captions [Line Items]                    
Debt Instrument, Interest Rate, Stated Percentage 5.50%               5.50%  
Repayments of debt $ 500