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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Summary of Signficant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2020
Jan. 01, 2019
Jan. 01, 2018
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes     $ 0        
Payments to Acquire Businesses, Gross $ 75            
Maximum              
Significant Accounting Policies [Line Items]              
Business Combination, Contingent Consideration, Liability $ 21 $ 21          
Aflac Japan              
Significant Accounting Policies [Line Items]              
Percentage of the Company's total revenues   69.00% 70.00% 70.00%      
Percentage of the Company's total assets 83.00% 83.00% 84.00%        
Accounting Standards Update 2016-02              
Significant Accounting Policies [Line Items]              
New accounting pronouncement or change in accounting principle, cumulative effect of change on assets           $ 134  
New accounting pronouncement or change in accounting principle, cumulative effect of change on liabilities           $ 134  
Accounting Standards Update 2019-04 | Forecast              
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         800    
Accumulated other comprehensive income (loss)              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes     $ 226        
Accumulated other comprehensive income (loss) | Accounting Standards Update 2016-01              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes             $ (148)
Accumulated other comprehensive income (loss) | Accounting Standards Update 2018-02              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes             374
Retained earnings              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes     $ (226)        
Retained earnings | Accounting Standards Update 2016-01              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes             148
Retained earnings | Accounting Standards Update 2018-02              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes             $ (374)
Retained earnings | Accounting Standards Update 2019-04 | Forecast              
Significant Accounting Policies [Line Items]              
Cumulative effect of change in accounting principle, net of income taxes         $ (56)