Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 2) ¥ in Millions, $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2020 JPY (¥)	Apr. 30, 2020 USD ($)	Mar. 31, 2020 JPY (¥)	Jan. 31, 2020	Dec. 31, 2019 USD ($)	Dec. 31, 2019 JPY (¥)	Sep. 30, 2019 JPY (¥)	Oct. 31, 2018 USD ($)	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 ($)
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 7,745					$ 6,408
4.00% senior notes paid January 2020
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate					4.00%	4.00%	4.00%											4.00%
Debt instrument, principal amount																		$ 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%										3.625%
Debt instrument, principal amount																	$ 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%									3.625%
Debt instrument, principal amount																$ 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%								3.25%
Debt instrument, principal amount															$ 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%							2.875%
Debt instrument, principal amount														$ 300
3.60% senior notes due April 2030
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.60%	3.60%	3.60%
Debt instrument, principal amount			$ 1,000
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%						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%						6.45%
Debt instrument, principal amount																			$ 450
4.00% senior noted due October 2046
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.00%	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%	4.75%
Debt instrument, principal amount									$ 550
.300% senior notes due September 2025
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.30%	0.30%		0.30%
Debt instrument, principal amount | ¥		¥ 12,400		¥ 12,400
.932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.932%	0.932%				0.932%	0.932%					0.932%
Debt instrument, principal amount | ¥		¥ 60,000					¥ 60,000					¥ 60,000
.500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.50%	0.50%				0.50%	0.50%
Debt instrument, principal amount | ¥		¥ 12,600					¥ 12,600
.550% senior notes due March 2030
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.55%	0.55%		0.55%
Debt instrument, principal amount | ¥		¥ 13,300		¥ 13,300
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%	1.159%
Debt instrument, principal amount | ¥		¥ 29,300					¥ 29,300			¥ 29,300
.843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.843%	0.843%				0.843%	0.843%
Debt instrument, principal amount | ¥		¥ 9,300					¥ 9,300
.750% senior notes due March 2032
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.75%	0.75%		0.75%
Debt instrument, principal amount | ¥		¥ 20,700		¥ 20,700
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%	1.488%
Debt instrument, principal amount | ¥		¥ 15,200					¥ 15,200			¥ 15,200
.934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.934%	0.934%				0.934%	0.934%
Debt instrument, principal amount | ¥		¥ 9,800					¥ 9,800
.830% senior notes due March 2035
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.83%	0.83%		0.83%
Debt instrument, principal amount | ¥		¥ 10,600		¥ 10,600
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%	1.75%
Debt instrument, principal amount | ¥		¥ 8,900					¥ 8,900			¥ 8,900
1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.122%	1.122%				1.122%	1.122%
Debt instrument, principal amount | ¥		¥ 6,300					¥ 6,300
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%				2.108%
Debt instrument, principal amount | ¥		¥ 60,000					¥ 60,000				¥ 60,000
Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.43%	0.43%				0.42%	0.42%
Debt instrument, principal amount | ¥		¥ 5,000					¥ 5,000	¥ 5,000
Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.58%	0.58%				0.57%	0.57%
Debt instrument, principal amount | ¥		¥ 25,000					¥ 25,000	¥ 25,000
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]
Debt instrument, interest rate					4.00%	4.00%	4.00%
Notes payable	$ 0					$ 348
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%	3.625%
Notes payable	$ 698					$ 698
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%	3.625%
Notes payable	$ 747					$ 747
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%	3.25%
Notes payable	$ 448					$ 448
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%	2.875%
Notes payable	$ 298					$ 298
Parent Company | 3.60% senior notes due April 2030
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	3.60%	3.60%	3.60%
Debt instrument, principal amount			$ 1,000
Notes payable	$ 990					$ 0
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%	6.90%
Notes payable	$ 221					$ 220
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%	6.45%
Notes payable	$ 254					$ 254
Parent Company | 4.00% senior noted due October 2046
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	4.00%	4.00%				4.00%	4.00%
Notes payable	$ 394					$ 394
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%	4.75%
Notes payable	$ 541					$ 541
Parent Company | .300% senior notes due September 2025
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.30%	0.30%		0.30%
Debt instrument, principal amount | ¥		¥ 12,400		¥ 12,400
Notes payable	$ 119					$ 0
Parent Company | .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 | ¥		¥ 60,000					¥ 60,000
Notes payable	$ 578					$ 545
Parent Company | .500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.50%	0.50%				0.50%	0.50%
Debt instrument, principal amount | ¥		¥ 12,600					¥ 12,600
Notes payable	$ 121					$ 114
Parent Company | .550% senior notes due March 2030
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.55%	0.55%		0.55%
Debt instrument, principal amount | ¥		¥ 13,300		¥ 13,300
Notes payable	$ 127					$ 0
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%	1.159%
Debt instrument, principal amount | ¥		¥ 29,300					¥ 29,300
Notes payable	$ 282					$ 266
Parent Company | .843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.843%	0.843%				0.843%	0.843%
Debt instrument, principal amount | ¥		¥ 9,300					¥ 9,300
Notes payable	$ 90					$ 84
Parent Company | .750% senior notes due March 2032
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.75%	0.75%		0.75%
Debt instrument, principal amount | ¥		¥ 20,700		¥ 20,700
Notes payable	$ 198					$ 0
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%	1.488%
Debt instrument, principal amount | ¥		¥ 15,200					¥ 15,200
Notes payable	$ 146					$ 138
Parent Company | .934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.934%	0.934%				0.934%	0.934%
Debt instrument, principal amount | ¥		¥ 9,800					¥ 9,800
Notes payable	$ 94					$ 88
Parent Company | .830% senior notes due March 2035
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.83%	0.83%		0.83%
Debt instrument, principal amount | ¥		¥ 10,600		¥ 10,600
Notes payable	$ 101					$ 0
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%	1.75%
Debt instrument, principal amount | ¥		¥ 8,900					¥ 8,900
Notes payable	$ 85					$ 81
Parent Company | 1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	1.122%	1.122%				1.122%	1.122%
Debt instrument, principal amount | ¥		¥ 6,300					¥ 6,300
Notes payable	$ 61					$ 57
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%	2.108%
Debt instrument, principal amount | ¥		¥ 60,000					¥ 60,000
Notes payable	$ 575					$ 543
Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.43%	0.43%				0.42%	0.42%
Debt instrument, principal amount | ¥		¥ 5,000					¥ 5,000
Notes payable	$ 48					$ 45
Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate	0.58%	0.58%				0.57%	0.57%
Debt instrument, principal amount | ¥		¥ 25,000					¥ 25,000
Notes payable	$ 240					$ 227