REINSURANCE - Effect of Reinsurance on Premiums and Benefits and Claims (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Effects of Reinsurance [Line Items]
Direct premium income	$ 4,776	$ 4,833
Net premium income	4,691	4,745
Direct benefits and claims	3,041	3,119
Benefits and claims, net	2,967	3,042
All other
Effects of Reinsurance [Line Items]
Ceded premiums	(15)	(15)
Assumed premiums earned	1	2
Ceded benefits and claims	(11)	(12)
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income	3,180	3,263
Aflac Japan \| Closed Block
Effects of Reinsurance [Line Items]
Ceded premiums	(121)	(129)
Assumed premiums earned	50	54
Ceded benefits and claims	(111)	(117)
Assumed benefits and claims from other companies	48	52
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims	10	12
Assumed benefits and claims from other companies	$ (10)	$ (12)

Amount of earned premiums assumed from other entities.

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