Schedule II - Aflac Incorporated (Parent Only) - Summary of Notes Payable (Detail 2) $ in Millions, ¥ in Billions	1 Months Ended
	Oct. 31, 2018 USD ($)	Oct. 31, 2017 JPY (¥)	Jan. 31, 2017 JPY (¥)	Sep. 30, 2016 USD ($)	Mar. 31, 2015 USD ($)	Nov. 30, 2014 USD ($)	Jun. 30, 2013 USD ($)	Feb. 29, 2012 USD ($)	Aug. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 JPY (¥)	Nov. 30, 2018 USD ($)	Oct. 31, 2018 JPY (¥)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 JPY (¥)	Dec. 31, 2016	Sep. 30, 2016 JPY (¥)
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 5,765				$ 5,267
2.40% senior notes paid November 2018
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate					2.40%								2.40%		2.40%	2.40%
Debt instrument, principal amount					$ 550								$ 550
Debt instrument, term					5 years
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%	4.00%
Debt instrument, principal amount								$ 350			$ 350
Debt instrument, term								10 years
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				$ 700
Debt instrument, term							10 years
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					$ 750
Debt instrument, term						10 years
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						$ 450
Debt instrument, term					10 years
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%		2.875%
Debt instrument, principal amount				$ 300
Debt instrument, term				10 years
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
Debt instrument, term										30 years
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
Debt instrument, term									30 years
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%	4.00%		4.00%
Debt instrument, principal amount				$ 400
Debt instrument, term				30 years
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%
Debt instrument, principal amount	$ 550
Debt instrument, term	30 years
.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 | ¥			¥ 60.0									¥ 60.0				¥ 60.0
Debt instrument, term			10 years
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 | ¥												¥ 29.3		¥ 29.3
Debt instrument, term	12 years
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 | ¥												¥ 15.2		¥ 15.2
Debt instrument, term	15 years
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 | ¥												¥ 8.9		¥ 8.9
Debt instrument, term	20 years
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 | ¥		¥ 60.0										¥ 60.0				¥ 60.0
Debt instrument, term		30 years
Yen-denominated loan variable interest rate due September 2021
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate											0.32%	0.32%			0.32%	0.32%
Debt instrument, principal amount | ¥												¥ 5.0				¥ 5.0		¥ 5.0
Debt instrument, term				5 years
Yen-denominated loan variable interest rate due September 2023
Condensed Financial Statements, Captions [Line Items]
Debt instrument, interest rate											0.47%	0.47%			0.47%	0.47%
Debt instrument, principal amount | ¥												¥ 25.0				¥ 25.0		¥ 25.0
Debt instrument, term				7 years
Parent Company
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 5,765				$ 5,267
Parent Company | 2.40% senior notes paid November 2018
Condensed Financial Statements, Captions [Line Items]
Notes payable											0				$ 548
Debt instrument, interest rate													2.40%		2.40%	2.40%
Debt instrument, principal amount													$ 550
Parent Company | 4.00% senior notes due February 2022
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 348				$ 348
Debt instrument, interest rate											4.00%	4.00%			4.00%	4.00%
Parent Company | 3.625% senior notes due June 2023
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 698				$ 697
Debt instrument, interest rate											3.625%	3.625%			3.625%	3.625%
Parent Company | 3.625% senior notes due November 2024
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 746				$ 745
Debt instrument, interest rate											3.625%	3.625%			3.625%	3.625%
Parent Company | 3.25% senior notes due March 2025
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 447				$ 446
Debt instrument, interest rate											3.25%	3.25%			3.25%	3.25%
Parent Company | 2.875% senior notes due October 2026
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 297				$ 297
Debt instrument, interest rate											2.875%	2.875%			2.875%	2.875%
Parent Company | 6.90% senior notes due December 2039
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 220				$ 220
Debt instrument, interest rate											6.90%	6.90%			6.90%	6.90%
Parent Company | 6.45% senior notes due August 2040
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 254				$ 254
Debt instrument, interest rate											6.45%	6.45%			6.45%	6.45%
Parent Company | 4.00% senior notes due October 2046
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 394				$ 394
Debt instrument, interest rate											4.00%	4.00%			4.00%	4.00%
Parent Company | 4.750% senior notes due January 2049
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 540				$ 0
Debt instrument, interest rate	4.75%										4.75%	4.75%		4.75%
Debt instrument, term	30 years
Parent Company | .932% senior notes due January 2027
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 538				$ 528
Debt instrument, interest rate											0.932%	0.932%			0.932%	0.932%
Debt instrument, principal amount | ¥												¥ 60.0				¥ 60.0
Parent Company | 1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 262				$ 0
Debt instrument, interest rate	1.159%										1.159%	1.159%		1.159%
Debt instrument, principal amount | ¥												¥ 29.3		¥ 29.3
Debt instrument, term	12 years
Parent Company | 1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 136				0
Debt instrument, interest rate	1.488%										1.488%	1.488%		1.488%
Debt instrument, principal amount | ¥												¥ 15.2		¥ 15.2
Debt instrument, term	15 years
Parent Company | 1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 79				0
Debt instrument, interest rate	1.75%										1.75%	1.75%		1.75%
Debt instrument, principal amount | ¥												¥ 8.9		¥ 8.9
Debt instrument, term	20 years
Parent Company | 2.108% subordinated notes due October 2047
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 536				$ 526
Debt instrument, interest rate											2.108%	2.108%			2.108%	2.108%
Debt instrument, principal amount | ¥												¥ 60.0				¥ 60.0
Parent Company | Yen-denominated loan variable interest rate due September 2021
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 45				$ 44
Debt instrument, interest rate											0.32%	0.32%			0.32%	0.32%
Debt instrument, principal amount | ¥												¥ 5.0				¥ 5.0
Parent Company | Yen-denominated loan variable interest rate due September 2023
Condensed Financial Statements, Captions [Line Items]
Notes payable											$ 225				$ 220
Debt instrument, interest rate											0.47%	0.47%			0.47%	0.47%
Debt instrument, principal amount | ¥												¥ 25.0				¥ 25.0