Schedule II - Aflac Incorporated (Parent Only) - Condensed Statement of Earnings (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, [Line Items]
Net investment income		$ 886	$ 936	$ 878	$ 878	$ 874	$ 870	$ 862	$ 837	$ 3,578	$ 3,442	$ 3,220
Realized investment gains (losses)										(135)	(430)	(151)
Total revenues		5,603	5,536	5,511	5,657	5,126	5,577	5,589	5,464	22,307	21,758	21,667
Interest expense										228	222	240
Income tax expense (benefit)		$ 276	$ 259	$ 292	$ 314	$ 197	$ 301	$ 299	$ 265	1,141	1,063	(586)
Net earnings										3,304	2,920	4,604
Parent Company
Condensed Income Statements, [Line Items]
Management and service fees from subsidiaries	[1]									151	190	297
Net investment income										77	69	30
Interest from subsidiaries	[1]									4	4	5
Realized investment gains (losses)										98	(16)	(1)
Total revenues										330	247	331
Interest expense										200	188	197
Other operating expenses										221	225	180	[2]
Total operating expenses										421	413	377
Earnings before income taxes and equity in earnings of subsidiaries										(91)	(166)	(46)
Income tax expense (benefit)										(22)	(12)	(23)
Earnings before equity in earnings of subsidiaries										(69)	(154)	(23)
Equity in earnings of subsidiaries	[1]									3,373	3,074	4,627
Net earnings										$ 3,304	$ 2,920	4,604
5.50% subordinated notes due September 2052
Condensed Income Statements, [Line Items]
Expense on extinguishment of debt												$ 13

[1] Eliminated in consolidation

[2] Includes expense of $13 in 2017 for the early extinguishment of debt