Schedule II - Aflac Incorporated (Parent Only) - Condensed Balance Sheet (Detail 2) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
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)	672,881	671,249	669,723	668,132
Parent Company
Condensed Financial Statements, Captions [Line Items]
Securities available for sale, fixed maturities, amortized cost	$ 1,163	$ 483
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)	672,881	671,249