REINSURANCE - Effect of Reinsurance on Premiums and Benefits and Claims (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Effects of Reinsurance [Line Items]
Direct premium income	$ 4,772	$ 4,776
Net premium income	4,681	4,691
Direct benefits and claims	3,028	3,041
Benefits and claims, net	2,939	2,967
All other
Effects of Reinsurance [Line Items]
Ceded premiums	(24)	(15)
Assumed premiums earned	0	1
Ceded benefits and claims	(25)	(11)
Assumed benefits and claims from other companies	0	0
Aflac Japan
Effects of Reinsurance [Line Items]
Net premium income	3,150	3,180
Aflac Japan \| Closed Block
Effects of Reinsurance [Line Items]
Ceded premiums	(116)	(121)
Assumed premiums earned	49	50
Ceded benefits and claims	(105)	(111)
Assumed benefits and claims from other companies	41	48
Intercompany eliminations
Effects of Reinsurance [Line Items]
Ceded benefits and claims	10	10
Assumed benefits and claims from other companies	$ (10)	$ (10)

Amount of earned premiums assumed from other entities.

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