SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION - (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 9,505				$ 8,993				$ 9,505	$ 8,993
Future policy benefits & unpaid policy claims	86,249				80,151				86,249	80,151
Unearned premiums	5,959				6,916				5,959	6,916
Other policyholders' funds	6,939				6,659				6,939	6,659
Premiums Earned, Net	4,580	$ 4,648	$ 4,665	$ 4,638	4,778	$ 5,022	$ 4,823	$ 4,602	18,531	19,225	$ 17,570
Net investment income	812	$ 811	$ 802	$ 794	813	$ 842	$ 822	$ 801	3,220	3,278	3,135
Benefits and claims, net									12,181	12,919	11,746
Amortization of deferred policy acquisition costs									1,132	1,141	1,066
Other operating expenses									4,336	4,432	4,198
Premiums written									17,657	18,214	17,083
Aflac Japan
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	6,150				5,765				6,150	5,765	5,370
Future policy benefits & unpaid policy claims	76,353				70,684				76,353	70,684
Unearned premiums	5,840				6,798				5,840	6,798
Other policyholders' funds	6,939				6,659				6,939	6,659
Premiums Earned, Net									12,752	13,537	12,046
Net investment income									2,463	2,554	2,436
Benefits and claims, net									9,087	9,828	8,705
Amortization of deferred policy acquisition costs									630	644	578
Other operating expenses									2,257	2,326	2,055
Premiums written									12,092	12,762	11,740
Aflac U.S.
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	3,355				3,228				3,355	3,228	3,141
Future policy benefits & unpaid policy claims	10,506				10,094				10,506	10,094
Unearned premiums	119				118				119	118
Other policyholders' funds	0				0				0	0
Premiums Earned, Net									5,563	5,454	5,347
Net investment income									721	703	678
Benefits and claims, net									2,885	2,869	2,873
Amortization of deferred policy acquisition costs									502	497	488
Other operating expenses									1,658	1,593	1,570
Premiums written									5,565	5,452	5,343
All other
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	0				0				0	0
Future policy benefits & unpaid policy claims	138				91				138	91
Unearned premiums	0				0				0	0
Other policyholders' funds	0				0				0	0
Premiums Earned, Net									216	234	177
Net investment income									36	21	21
Benefits and claims, net									209	222	168
Amortization of deferred policy acquisition costs									0	0	0
Other operating expenses									421	513	573
Premiums written									0	0	$ 0
Intercompany eliminations
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	0				0				0	0
Future policy benefits & unpaid policy claims	(748)				(718)				(748)	(718)
Unearned premiums	0				0				0	0
Other policyholders' funds	$ 0				$ 0				$ 0	$ 0