REINSURANCE Effect of Reinsurance on Premiums and Benefits and Claims (Details) - 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
Effects of Reinsurance [Line Items]
Direct premium income									$ 18,875	$ 19,592	$ 17,904
Ceded premiums									(566)	(608)	(520)
Assumed premiums earned									222	241	186
Net premium income	$ 4,580	$ 4,648	$ 4,665	$ 4,638	$ 4,778	$ 5,022	$ 4,823	$ 4,602	18,531	19,225	17,570
Direct benefits and claims									12,486	13,240
Benefits and claims, net									12,181	12,919	11,746
All other
Effects of Reinsurance [Line Items]
Ceded premiums									(51)	(48)
Assumed premiums earned									6	7
Ceded benefits and claims									(44)	(38)
Assumed benefits and claims from other companies									3	4
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income									12,752	13,537	12,046
Benefits and claims, net									9,087	9,828	$ 8,705
Aflac Japan | Closed block
Effects of Reinsurance [Line Items]
Ceded premiums									(515)	(560)
Assumed premiums earned									216	234
Ceded benefits and claims									(473)	(509)
Assumed benefits and claims from other companies									209	222
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims									51	58
Assumed benefits and claims from other companies									$ (51)	$ (58)