Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 1) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Notes payable		$ 6,408	$ 5,765
Parent Company
Condensed Financial Statements, Captions [Line Items]
Notes payable		6,136	5,765
Parent Company | 4.00% senior notes due February 2022
Condensed Financial Statements, Captions [Line Items]
Notes payable	[1]	348	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	746
Parent Company | 3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Notes payable		448	447
Parent Company | 2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable		298	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		541	540
Parent Company | .932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Notes payable		545	538
Parent Company | .500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Notes payable		114	0
Parent Company | 1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable		266	262
Parent Company | .843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Notes payable		84	0
Parent Company | 1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Notes payable		138	136
Parent Company | .934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Notes payable		88	0
Parent Company | 1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Notes payable		81	79
Parent Company | 1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Notes payable		57	0
Parent Company | 2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Notes payable		543	536
Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable		45	45
Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Notes payable		$ 227	$ 225

[1] Redeemed in January 2020