REINSURANCE Effect of Reinsurance on Premiums and Benefits and Claims (Details) - 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
Effects of Reinsurance [Line Items]
Direct premium income									$ 19,018	$ 18,875	$ 19,592
Ceded premiums									(555)	(566)	(608)
Assumed premiums earned									214	222	241
Net premium income	$ 4,591	$ 4,636	$ 4,706	$ 4,745	$ 4,580	$ 4,648	$ 4,665	$ 4,638	18,677	18,531	19,225
Direct benefits and claims									12,293	12,486	13,240
Benefits and claims, net									12,000	12,181	12,919
All other
Effects of Reinsurance [Line Items]
Ceded premiums									(58)	(51)	(48)
Assumed premiums earned									6	6	7
Ceded benefits and claims									(44)	(44)	(38)
Assumed benefits and claims from other companies									2	3	4
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income									12,762	12,752	13,537
Benefits and claims, net									8,913	9,087	9,828
Aflac Japan | Closed block
Effects of Reinsurance [Line Items]
Ceded premiums									(497)	(515)	(560)
Assumed premiums earned									208	216	234
Ceded benefits and claims									(450)	(473)	(509)
Assumed benefits and claims from other companies									209	209	222
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims									43	51	58
Assumed benefits and claims from other companies									$ (53)	$ (51)	$ (58)