REINSURANCE - Effect of Reinsurance on Premiums and Benefits and Claims (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Effects of Reinsurance [Line Items]
Direct premium income	$ 4,720	$ 4,734	$ 14,346	$ 14,210
Net premium income	4,636	4,648	14,086	13,951
Direct benefits and claims	3,076	3,157	9,301	9,403
Benefits and claims, net	3,002	3,083	9,075	9,174
All other
Effects of Reinsurance [Line Items]
Ceded premiums	(15)	(12)	(45)	(37)
Assumed premiums earned	2	2	5	5
Ceded benefits and claims	(11)	(9)	(34)	(31)
Assumed benefits and claims from other companies	0	0	2	2
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income	3,159	3,200	9,649	9,616
Aflac Japan \| Closed Block
Effects of Reinsurance [Line Items]
Ceded premiums	(122)	(130)	(378)	(390)
Assumed premiums earned	51	54	158	163
Ceded benefits and claims	(111)	(118)	(343)	(359)
Assumed benefits and claims from other companies	48	53	149	159
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims	10	13	33	39
Assumed benefits and claims from other companies	$ (10)	$ (13)	$ (33)	$ (39)

Amount of earned premiums assumed from other entities.

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