REINSURANCE - Effect of Reinsurance on Premiums and Benefits and Claims (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Effects of Reinsurance [Line Items]
Direct premium income	$ 4,823	$ 4,720	$ 14,365	$ 14,346
Net premium income	4,736	4,636	14,109	14,086
Direct benefits and claims	3,106	3,076	9,185	9,301
Benefits and claims, net	3,027	3,002	8,958	9,075
All other
Effects of Reinsurance [Line Items]
Ceded premiums	(17)	(15)	(50)	(45)
Assumed premiums earned	0	2	4	5
Ceded benefits and claims	(18)	(11)	(41)	(34)
Assumed benefits and claims from other companies	1	0	1	2
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income	3,241	3,159	9,593	9,649
Aflac Japan \| Closed Block
Effects of Reinsurance [Line Items]
Ceded premiums	(121)	(122)	(361)	(378)
Assumed premiums earned	51	51	151	158
Ceded benefits and claims	(111)	(111)	(331)	(343)
Assumed benefits and claims from other companies	49	48	144	149
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims	11	10	31	33
Assumed benefits and claims from other companies	$ (11)	$ (10)	$ (31)	$ (33)

Amount of earned premiums assumed from other entities.

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