Schedule II - Aflac Incorporated (Parent Only) - Condensed Statement of Earnings (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Income Statements, [Line Items]
Net investment income		$ 968	$ 896	$ 870	$ 904	$ 886	$ 936	$ 878	$ 878	$ 3,638	$ 3,578	$ 3,442
Net investment gains (losses)		256	108	(170)	(463)	12	(153)	(66)	71	(270)	(135)	(430)
Total revenues		5,913	5,665	5,407	5,162	5,603	5,536	5,511	5,657	22,147	22,307	21,758
Interest expense										242	228	222
Income tax expense (benefit)		$ 265	$ (1,303)	$ 265	$ 154	$ 276	$ 259	$ 292	$ 314	(619)	1,141	1,063
Net earnings										4,778	3,304	2,920
Parent Company
Condensed Income Statements, [Line Items]
Management and service fees from subsidiaries	[1]									131	151	190
Net investment income										62	77	69
Interest from subsidiaries	[1]									3	4	4
Net investment gains (losses)										399	98	(16)
Total revenues										595	330	247
Interest expense										221	200	188
Other operating expenses	[2]									277	221	225
Total operating expenses										498	421	413
Earnings before income taxes and equity in earnings of subsidiaries										97	(91)	(166)
Income tax expense (benefit)										(15)	(22)	(12)
Earnings before equity in earnings of subsidiaries										112	(69)	(154)
Equity in earnings of subsidiaries	[1]									4,666	3,373	3,074
Net earnings										4,778	$ 3,304	$ 2,920
4.00% senior notes paid January 2020
Condensed Income Statements, [Line Items]
Expense on extinguishment of debt										15
4.00% senior notes paid January 2020 | Parent Company
Condensed Income Statements, [Line Items]
Expense on extinguishment of debt										$ 15

[1] Eliminated in consolidation

[2] Includes expense of $15 in 2020 for the early extinguishment of debt