Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 2) $ in Millions, ¥ in Billions	Dec. 31, 2019 USD ($)	Dec. 31, 2019 JPY (¥)	Sep. 30, 2019 JPY (¥)	Dec. 31, 2018 JPY (¥)	Oct. 31, 2018 USD ($)	Oct. 31, 2018 JPY (¥)	Oct. 31, 2017 JPY (¥)	Jan. 31, 2017 JPY (¥)	Dec. 31, 2016	Sep. 30, 2016 USD ($)	Mar. 31, 2015 USD ($)	Nov. 30, 2014 USD ($)	Jun. 30, 2013 USD ($)	Feb. 29, 2012 USD ($)	Aug. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)
4.00% senior notes due February 2022
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.00%	4.00%		4.00%										4.00%
Debt instrument, principal amount | $	$ 350													$ 350
3.625% senior notes due June 2023
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.625%	3.625%		3.625%									3.625%
Debt instrument, principal amount | $	$ 700												$ 700
3.625% senior notes due November 2024
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.625%	3.625%		3.625%								3.625%
Debt instrument, principal amount | $	$ 750											$ 750
3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.25%	3.25%		3.25%							3.25%
Debt instrument, principal amount | $	$ 450										$ 450
2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	2.875%	2.875%		2.875%						2.875%
Debt instrument, principal amount | $										$ 300
6.90% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	6.90%	6.90%		6.90%					6.90%
Debt instrument, principal amount | $																$ 400
6.45% senior notes due August 2040
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	6.45%	6.45%		6.45%					6.45%
Debt instrument, principal amount | $															$ 450
4.00% senior notes due October 2046
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.00%	4.00%		4.00%						4.00%
Debt instrument, principal amount | $										$ 400
4.750% senior notes due January 2049
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.75%	4.75%		4.75%	4.75%	4.75%
Debt instrument, principal amount | $					$ 550
.932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.932%	0.932%		0.932%				0.932%
Debt instrument, principal amount		¥ 60.0		¥ 60.0				¥ 60.0
.500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.50%	0.50%
Debt instrument, principal amount		¥ 12.6
1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.159%	1.159%		1.159%	1.159%	1.159%
Debt instrument, principal amount		¥ 29.3		¥ 29.3		¥ 29.3
.843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.843%	0.843%
Debt instrument, principal amount		¥ 9.3
1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.488%	1.488%		1.488%	1.488%	1.488%
Debt instrument, principal amount		¥ 15.2		¥ 15.2		¥ 15.2
.934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.934%	0.934%
Debt instrument, principal amount		¥ 9.8
1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.75%	1.75%		1.75%	1.75%	1.75%
Debt instrument, principal amount		¥ 8.9		¥ 8.9		¥ 8.9
1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.122%	1.122%
Debt instrument, principal amount		¥ 6.3
2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	2.108%	2.108%		2.108%			2.108%
Debt instrument, principal amount		¥ 60.0		¥ 60.0			¥ 60.0
Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.42%	0.42%		0.32%
Debt instrument, principal amount		¥ 5.0	¥ 5.0	¥ 5.0
Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.57%	0.57%		0.47%
Debt instrument, principal amount		¥ 25.0	25.0	¥ 25.0
Parent Company | 4.00% senior notes due February 2022
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.00%	4.00%		4.00%
Parent Company | 3.625% senior notes due June 2023
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.625%	3.625%		3.625%
Parent Company | 3.625% senior notes due November 2024
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.625%	3.625%		3.625%
Parent Company | 3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.25%	3.25%		3.25%
Parent Company | 2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	2.875%	2.875%		2.875%
Parent Company | 6.90% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	6.90%	6.90%		6.90%
Parent Company | 6.45% senior notes due August 2040
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	6.45%	6.45%		6.45%
Parent Company | 4.00% senior notes due October 2046
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.00%	4.00%		4.00%
Parent Company | 4.750% senior notes due January 2049
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.75%	4.75%		4.75%
Parent Company | .932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.932%	0.932%		0.932%
Debt instrument, principal amount		¥ 60.0		¥ 60.0
Parent Company | .500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.50%	0.50%
Debt instrument, principal amount		¥ 12.6
Parent Company | 1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.159%	1.159%		1.159%
Debt instrument, principal amount		¥ 29.3		¥ 29.3
Parent Company | .843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.843%	0.843%
Debt instrument, principal amount		¥ 9.3
Parent Company | 1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.488%	1.488%		1.488%
Debt instrument, principal amount		¥ 15.2		¥ 15.2
Parent Company | .934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.934%	0.934%
Debt instrument, principal amount		¥ 9.8
Parent Company | 1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.75%	1.75%		1.75%
Debt instrument, principal amount		¥ 8.9		¥ 8.9
Parent Company | 1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.122%	1.122%
Debt instrument, principal amount		¥ 6.3
Parent Company | 2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	2.108%	2.108%		2.108%
Debt instrument, principal amount		¥ 60.0		¥ 60.0
Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.42%	0.42%		0.32%
Debt instrument, principal amount		¥ 5.0	5.0	¥ 5.0
Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.57%	0.57%		0.47%
Debt instrument, principal amount		¥ 25.0	¥ 25.0	¥ 25.0