Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 1) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 5,267		$ 5,339
Parent Company
Condensed Financial Statements, Captions [Line Items]
Notes payable	5,267		5,339
Parent Company | 2.65% senior notes paid February 2017
Condensed Financial Statements, Captions [Line Items]
Notes payable	0		649
Parent Company | 2.40% senior notes due March 2020
Condensed Financial Statements, Captions [Line Items]
Notes payable	548		547
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	697		696
Parent Company | 3.625% senior notes due November 2024
Condensed Financial Statements, Captions [Line Items]
Notes payable	745		745
Parent Company | 3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Notes payable	446		445
Parent Company | 2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable	297		298
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 | 5.50% subordinated notes due September 2052
Condensed Financial Statements, Captions [Line Items]
Notes payable	0	[1]	486
Parent Company | .932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Notes payable	528		0
Parent Company | 2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Notes payable	526		0
Parent Company | Yen-denominated loan variable interest rate due September 2021
Condensed Financial Statements, Captions [Line Items]
Notes payable	44		43
Parent Company | Yen-denominated loan variable interest rate due September 2023
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 220		$ 214

[1] Redeemed in November 2017