Schedule II - Aflac Incorporated (Parent Only) - Condensed Balance Sheet (Detail 2) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Available for sale, fixed maturity securities, amortized cost	$ 76,856	[1],[2]	$ 75,132	[3]
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)	1,347,540		1,345,762		1,342,498	1,339,446
Parent Company
Condensed Financial Statements, Captions [Line Items]
Available for sale, fixed maturity securities, amortized cost	$ 1,209		$ 1,163
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)	1,347,540		1,345,762

[1] Includes perpetual securities ($1,139 at amortized cost and $1,140 at fair value)

[2] Includes perpetual securities, categorized in accordance with their respective economic maturities (the expected maturity date created by the combination of features in the financial instrument)

[3] Includes perpetual securities ($1,462 at amortized cost and $1,789 at fair value)