Schedule II - Aflac Incorporated (Parent Only) - Additional Information (Detail) $ in Millions, ¥ in Billions	1 Months Ended
	Oct. 31, 2018 USD ($) series	Mar. 31, 2015 USD ($) series	Dec. 31, 2018 USD ($)	Dec. 31, 2018 JPY (¥)	Nov. 30, 2018 USD ($)	Oct. 31, 2018 JPY (¥) series	Dec. 31, 2017 USD ($)	Sep. 30, 2016 USD ($) series
Condensed Financial Statements, Captions [Line Items]
Notes payable			$ 5,778				$ 5,289
Number of series of senior notes issued through a U.S. public debt offering (in series) | series	3	2				3		2
4.750% senior notes due January 2049
Condensed Financial Statements, Captions [Line Items]
Notes payable			$ 540				0
Debt Instrument, Face Amount	$ 550
Debt Instrument, Interest Rate, Stated Percentage	4.75%		4.75%	4.75%		4.75%
Debt instrument, term	30 years
Debt instrument, redemption, description	These notes are redeemable at the Parent Company's option in whole at any time or in part from time to time at a redemption price equal to the greater of: (i) the aggregate principal amount of the notes to be redeemed or (ii) the amount equal to the sum of the present values of the remaining scheduled payments for principal of and interest on the notes to be redeemed, not including any portion of the payments of interest accrued as of such redemption date, discounted to such redemption date on a semiannual basis at the yield to maturity for a United States Treasury security with a maturity comparable to the remaining term of the notes, plus 25 basis points, plus in each case, accrued and unpaid interest on the principal amount of the notes to be redeemed to, but excluding, such redemption date.
2.40% senior notes paid November 2018
Condensed Financial Statements, Captions [Line Items]
Notes payable			$ 0				$ 548
Debt Instrument, Face Amount		$ 550			$ 550
Debt Instrument, Interest Rate, Stated Percentage		2.40%			2.40%		2.40%
Debt instrument, term		5 years
Senior Notes
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Face Amount		$ 1,000				¥ 53.4		$ 700
1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Notes payable			$ 262				$ 0
Debt Instrument, Face Amount | ¥				¥ 29.3		¥ 29.3
Debt Instrument, Interest Rate, Stated Percentage	1.159%		1.159%	1.159%		1.159%
Debt instrument, term	12 years
1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Notes payable			$ 136				0
Debt Instrument, Face Amount | ¥				¥ 15.2		¥ 15.2
Debt Instrument, Interest Rate, Stated Percentage	1.488%		1.488%	1.488%		1.488%
Debt instrument, term	15 years
1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Notes payable			$ 79				$ 0
Debt Instrument, Face Amount | ¥				¥ 8.9		¥ 8.9
Debt Instrument, Interest Rate, Stated Percentage	1.75%		1.75%	1.75%		1.75%
Debt instrument, term	20 years
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	3					3
Parent Company | 4.750% senior notes due January 2049
Condensed Financial Statements, Captions [Line Items]
Notes payable	$ 550
Debt Instrument, Interest Rate, Stated Percentage	4.75%		4.75%	4.75%		4.75%
Debt instrument, term	30 years
Debt instrument, redemption, description	These notes are redeemable at the Parent Company's option in whole at any time or in part from time to time at a redemption price equal to the greater of: (i) the aggregate principal amount of the notes to be redeemed or (ii) the amount equal to the sum of the present values of the remaining scheduled payments for principal of and interest on the notes to be redeemed, not including any portion of the payments of interest accrued as of such redemption date, discounted to such redemption date on a semiannual basis at the yield to maturity for a United States Treasury security with a maturity comparable to the remaining term of the notes, plus 25 basis points, plus in each case, accrued and unpaid interest on the principal amount of the notes to be redeemed to, but excluding, such redemption date.
Parent Company | 2.40% senior notes paid November 2018
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Face Amount					$ 550
Debt Instrument, Interest Rate, Stated Percentage					2.40%		2.40%
Parent Company | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Face Amount | ¥						¥ 53.4
Parent Company | 1.159% senior notes due October 2030
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Face Amount | ¥				¥ 29.3		¥ 29.3
Debt Instrument, Interest Rate, Stated Percentage	1.159%		1.159%	1.159%		1.159%
Debt instrument, term	12 years
Parent Company | 1.488% senior notes due October 2033
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Face Amount | ¥				¥ 15.2		¥ 15.2
Debt Instrument, Interest Rate, Stated Percentage	1.488%		1.488%	1.488%		1.488%
Debt instrument, term	15 years
Parent Company | 1.750% senior notes due October 2038
Condensed Financial Statements, Captions [Line Items]
Debt Instrument, Face Amount | ¥				¥ 8.9		¥ 8.9
Debt Instrument, Interest Rate, Stated Percentage	1.75%		1.75%	1.75%		1.75%
Debt instrument, term	20 years