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Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Millions, ¥ in Billions
3 Months Ended
Mar. 31, 2020
JPY (¥)
Mar. 31, 2019
Apr. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Jan. 01, 2020
USD ($)
Dec. 31, 2019
USD ($)
Senior notes            
Significant Accounting Policies [Line Items]            
Debt instrument, principal amount | ¥ ¥ 57.0          
Aflac Japan            
Significant Accounting Policies [Line Items]            
Percentage of the Company's total assets 84.00%     84.00%   84.00%
Percentage of the Company's total revenues 67.00% 68.00%        
Accounting Standards Update 2016-13            
Significant Accounting Policies [Line Items]            
Cumulative effect of change in accounting principle, net of income taxes [1]           $ (56)
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  
New Accounting Pronouncement Or Change In Accounting Principle Effect Of Adoption Unrealized Gain Loss Net Of Tax Reclassification To Accumulated Other Comprehensive Income         848  
Cumulative effect of change in accounting principle, net of income taxes [1]           848
Unrealized gains (losses) on investment securities | Accounting Standards Update 2019-04            
Significant Accounting Policies [Line Items]            
Cumulative effect of change in accounting principle, net of income taxes         848  
Retained earnings | Accounting Standards Update 2016-13            
Significant Accounting Policies [Line Items]            
Cumulative effect of change in accounting principle, net of income taxes         $ (56) (56) [1]
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
Subsequent event | 3.60% senior notes due April 2030            
Significant Accounting Policies [Line Items]            
Debt instrument, principal amount     $ 1,000      
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    
[1] See Note 1 of the Notes to the Consolidated Financial Statements for the adoption of accounting guidance on January 1, 2020