Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 1) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 7,745	$ 6,408
Parent Company
Condensed Financial Statements, Captions [Line Items]
Notes payable	7,456	6,136
Parent Company | 4.00% senior notes paid January 2020
Condensed Financial Statements, Captions [Line Items]
Notes payable	0	348
Parent Company | 3.625% senior notes due June 2023
Condensed Financial Statements, Captions [Line Items]
Notes payable	698	698
Parent Company | 3.625% senior notes due November 2024
Condensed Financial Statements, Captions [Line Items]
Notes payable	747	747
Parent Company | 3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Notes payable	448	448
Parent Company | 2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable	298	298
Parent Company | 3.60% senior notes due April 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable	990	0
Parent Company | 6.90% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Notes payable	221	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 noted 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	541	541
Parent Company | .300% senior notes due September 2025
Condensed Financial Statements, Captions [Line Items]
Notes payable	119	0
Parent Company | .932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Notes payable	578	545
Parent Company | .500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Notes payable	121	114
Parent Company | .550% senior notes due March 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable	127	0
Parent Company | 1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable	282	266
Parent Company | .843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Notes payable	90	84
Parent Company | .750% senior notes due March 2032
Condensed Financial Statements, Captions [Line Items]
Notes payable	198	0
Parent Company | 1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Notes payable	146	138
Parent Company | .934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Notes payable	94	88
Parent Company | .830% senior notes due March 2035
Condensed Financial Statements, Captions [Line Items]
Notes payable	101	0
Parent Company | 1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Notes payable	85	81
Parent Company | 1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Notes payable	61	57
Parent Company | 2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Notes payable	575	543
Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable	48	45
Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 240	$ 227