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Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions, ¥ in Billions
9 Months Ended
Sep. 30, 2020
USD ($)
segment
Sep. 30, 2019
USD ($)
Sep. 30, 2020
JPY (¥)
Jun. 30, 2020
USD ($)
Apr. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
series
Mar. 31, 2020
JPY (¥)
series
Jan. 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Significant Accounting Policies [Line Items]                        
Number of Series of Senior Notes Issued Through a U.S. Public Debt Offering | series           4 4          
Cumulative effect of change in accounting principle, net of income taxes $ 32,479 $ 29,438   $ 29,420   $ 26,402     $ 28,959 $ 28,241 $ 26,049 $ 23,462
Number of reportable insurance business segments | segment 2                      
Senior notes                        
Significant Accounting Policies [Line Items]                        
Debt instrument, principal amount | ¥             ¥ 57.0          
3.60% senior notes due April 2030                        
Significant Accounting Policies [Line Items]                        
Debt instrument, principal amount $ 1,000       $ 1,000              
Aflac Japan                        
Significant Accounting Policies [Line Items]                        
Percentage of the Company's total assets 83.00%   83.00%           84.00%      
Percentage of the Company's total revenues 68.00% 69.00%                    
Accounting Standards Update 2019-04                        
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        
Retained earnings                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes $ 37,460 $ 33,710   35,204   34,599     $ 34,291 33,130 32,513 31,788
Accumulated other comprehensive income (loss)                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes 7,917 $ 7,251   $ 6,750   $ 4,188     6,615 $ 6,364 $ 4,481 $ 2,151
100.0 billion yen line of credit                        
Significant Accounting Policies [Line Items]                        
Line of credit facility, maximum borrowing capacity | ¥     ¥ 100.0                  
$1 billion line of credit                        
Significant Accounting Policies [Line Items]                        
Line of credit facility, maximum borrowing capacity $ 1,000                      
Federal Home Loan Bank of Atlanta                        
Significant Accounting Policies [Line Items]                        
Line of credit facility, maximum borrowing capacity         800              
Line of credit facility, capacity designated for short-term liquidity         $ 300              
Cumulative effect, period of adoption, adjustment | Accounting Standards Update 2016-13                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes [1]                 (56)      
Cumulative effect, period of adoption, adjustment | Accounting Standards Update 2019-04                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes [1]                 848      
Cumulative effect, period of adoption, adjustment | Retained earnings | Accounting Standards Update 2016-13                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes [1]                 (56)      
Cumulative effect, period of adoption, adjustment | Retained earnings | Accounting Standards Update 2019-04                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes [1]                 0      
Cumulative effect, period of adoption, adjustment | Accumulated other comprehensive income (loss) | Accounting Standards Update 2016-13                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes [1]                 0      
Cumulative effect, period of adoption, adjustment | Accumulated other comprehensive income (loss) | Accounting Standards Update 2019-04                        
Significant Accounting Policies [Line Items]                        
Cumulative effect of change in accounting principle, net of income taxes [1]                 $ 848      
[1] See Note 1 of the Notes to the Consolidated Financial Statements for the adoption of accounting guidance on January 1, 2020.