Schedule II - Aflac Incorporated (Parent Only) - Condensed Balance Sheet (Detail 2) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Available for sale, fixed maturity securities, amortized cost	$ 91,630	$ 79,371
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	1,900,000	1,900,000
Common stock, shares issued (in shares)	1,351,018	1,349,309	1,347,540	1,345,762
Parent Company
Condensed Financial Statements, Captions [Line Items]
Available for sale, fixed maturity securities, amortized cost	$ 1,782	$ 1,506
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	1,900,000	1,900,000
Common stock, shares issued (in shares)	1,351,018	1,349,309