Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 1) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 5,765	$ 5,267
Parent Company
Condensed Financial Statements, Captions [Line Items]
Notes payable	5,765	5,267
Parent Company | 2.40% senior notes paid November 2018
Condensed Financial Statements, Captions [Line Items]
Notes payable	0	548
Parent Company | 4.00% senior notes due February 2022
Condensed Financial Statements, Captions [Line Items]
Notes payable	348	348
Parent Company | 3.625% senior notes due June 2023
Condensed Financial Statements, Captions [Line Items]
Notes payable	698	697
Parent Company | 3.625% senior notes due November 2024
Condensed Financial Statements, Captions [Line Items]
Notes payable	746	745
Parent Company | 3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Notes payable	447	446
Parent Company | 2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable	297	297
Parent Company | 6.90% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Notes payable	220	220
Parent Company | 6.45% senior notes due August 2040
Condensed Financial Statements, Captions [Line Items]
Notes payable	254	254
Parent Company | 4.00% senior notes due October 2046
Condensed Financial Statements, Captions [Line Items]
Notes payable	394	394
Parent Company | 4.750% senior notes due January 2049
Condensed Financial Statements, Captions [Line Items]
Notes payable	540	0
Parent Company | .932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Notes payable	538	528
Parent Company | 1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable	262	0
Parent Company | 1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Notes payable	136	0
Parent Company | 1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Notes payable	79	0
Parent Company | 2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Notes payable	536	526
Parent Company | Yen-denominated loan variable interest rate due September 2021
Condensed Financial Statements, Captions [Line Items]
Notes payable	45	44
Parent Company | Yen-denominated loan variable interest rate due September 2023
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 225	$ 220