Schedule II - Aflac Incorporated (Parent Only) - Condensed Statement of Earnings (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, [Line Items]
Net investment income		$ 874	$ 870	$ 862	$ 837	$ 812	$ 811	$ 802	$ 794	$ 3,442	$ 3,220	$ 3,278
Realized investment gains (losses)										(430)	(151)	(14)
Other income (loss)		16	15	18	16	17	17	17	17	69	67	70
Total revenues		5,126	5,577	5,589	5,464	5,424	5,506	5,428	5,309	21,758	21,667	22,559
Interest expense										222	240	268
Current tax expense (benefit)										1,379	631	884
Deferred tax expense (benefit)										(316)	(1,217)	524
Income tax expense (benefit)		$ 197	$ 301	$ 299	$ 265	$ (1,585)	$ 359	$ 332	$ 306	1,063	(586)	1,408
Net earnings										2,920	4,604	2,659
Parent Company
Condensed Income Statements, [Line Items]
Management and service fees from subsidiaries	[1]									190	297	265
Net investment income										69	30	18
Interest from subsidiaries	[1]									4	5	5
Realized investment gains (losses)										90	67	84
Change in fair value on the cross-currency interest rate swaps										(106)	(68)	(159)
Total revenues										247	331	213
Interest expense										188	197	213
Other operating expenses	[2]									225	180	277
Total operating expenses										413	377	490
Earnings before income taxes and equity in earnings of subsidiaries										(166)	(46)	(277)
Income tax expense (benefit)										(12)	(23)	(102)
Earnings before equity in earnings of subsidiaries										(154)	(23)	(175)
Equity in earnings of subsidiaries	[1]									3,074	4,627	2,834
Net earnings										$ 2,920	4,604	2,659
5.50% subordinated notes due September 2052
Condensed Income Statements, [Line Items]
Expense on extinguishment of debt											$ 13
Senior Notes due 2039 and 2040
Condensed Income Statements, [Line Items]
Expense on extinguishment of debt												$ 137

[1] Eliminated in consolidation

[2] Includes expense of $13 in 2017 and $137 in 2016 for the early extinguishment of debt