Schedule II - Aflac Incorporated (Parent Only) - Additional Information (Detail) $ in Millions, ¥ in Billions	1 Months Ended
	Sep. 30, 2019 JPY (¥)	Dec. 31, 2019 JPY (¥) series	Dec. 31, 2018 JPY (¥)	Oct. 31, 2018 JPY (¥) series	Sep. 30, 2016 USD ($) series
Condensed Financial Statements, Captions [Line Items]
Number of series of senior notes issued through a U.S. public debt offering (in series) | series		4		3	2
Senior Notes
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 38.0		¥ 53.4	$ 700
.500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 12.6
Debt instrument, interest rate		0.50%
.843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 9.3
Debt instrument, interest rate		0.843%
.934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 9.8
Debt instrument, interest rate		0.934%
1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 6.3
Debt instrument, interest rate		1.122%
Yen-denominated loans
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount	¥ 30.0
Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount	¥ 5.0	¥ 5.0	¥ 5.0
Debt instrument, interest rate		0.42%	0.32%
Debt instrument, description of variable rate basis	bears interest at a rate per annum equal to the Tokyo interbank market rate (TIBOR), or alternate TIBOR, if applicable, plus the applicable TIBOR margin
Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount	¥ 25.0	¥ 25.0	¥ 25.0
Debt instrument, interest rate		0.57%	0.47%
Debt instrument, description of variable rate basis	bears interest at a rate per annum equal to the TIBOR, or alternate TIBOR, if applicable, plus the applicable TIBOR margin
Parent Company
Condensed Financial Statements, Captions [Line Items]
Number of series of senior notes issued through a U.S. public debt offering (in series) | series		4
Parent Company | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 38.0
Parent Company | .500% senior notes due December 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 12.6
Debt instrument, interest rate		0.50%
Parent Company | .843% senior notes due December 2031
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 9.3
Debt instrument, interest rate		0.843%
Parent Company | .934% senior notes due December 2034
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 9.8
Debt instrument, interest rate		0.934%
Parent Company | 1.122% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount		¥ 6.3
Debt instrument, interest rate		1.122%
Parent Company | Yen-denominated loans
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount	¥ 30.0
Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount	¥ 5.0	¥ 5.0	¥ 5.0
Debt instrument, interest rate		0.42%	0.32%
Debt instrument, description of variable rate basis	bears interest at a rate per annum equal to the Tokyo interbank market rate (TIBOR), or alternate TIBOR, if applicable, plus the applicable TIBOR margin
Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, principal amount	¥ 25.0	¥ 25.0	¥ 25.0
Debt instrument, interest rate		0.57%	0.47%
Debt instrument, description of variable rate basis	bears interest at a rate per annum equal to the TIBOR, or alternate TIBOR, if applicable, plus the applicable TIBOR margin
Lower Limit | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	0.30%
Lower Limit | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	0.45%
Lower Limit | Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	0.30%
Lower Limit | Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	0.45%
Upper Limit | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	0.70%
Upper Limit | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Upper Limit | Parent Company | Yen-denominated loan variable interest rate due September 2026
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	0.70%
Upper Limit | Parent Company | Yen-denominated loan variable interest rate due September 2029
Condensed Financial Statements, Captions [Line Items]
Debt instrument, basis spread on variable rate	1.00%