REINSURANCE Effect of Reinsurance on Premiums and Benefits and Claims (Details) - 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
Effects of Reinsurance [Line Items]
Direct premium income									$ 19,122	$ 19,018	$ 18,875
Ceded premiums									(547)	(555)	(566)
Assumed premiums earned									205	214	222
Net premium income	$ 4,671	$ 4,736	$ 4,681	$ 4,691	$ 4,591	$ 4,636	$ 4,706	$ 4,745	18,780	18,677	18,531
Direct benefits and claims									12,237	12,293	12,486
Benefits and claims, net									11,942	12,000	12,181
All other
Effects of Reinsurance [Line Items]
Ceded premiums									(69)	(58)	(51)
Assumed premiums earned									5	6	6
Ceded benefits and claims									(57)	(44)	(44)
Assumed benefits and claims from other companies									1	2	3
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income									12,772	12,762	12,752
Benefits and claims, net									8,877	8,913	9,087
Aflac Japan | Closed block
Effects of Reinsurance [Line Items]
Ceded premiums									(478)	(497)	(515)
Assumed premiums earned									200	208	216
Ceded benefits and claims									(433)	(450)	(473)
Assumed benefits and claims from other companies									194	209	209
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims									41	43	51
Assumed benefits and claims from other companies									$ (41)	$ (53)	$ (51)