Schedule II - Aflac Incorporated (Parent Only) - Supplemental Disclosure of Cash Flow Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Interest paid	$ 196	$ 211	$ 236
Shareholder dividend reinvestment
Condensed Cash Flow Statements, Captions [Line Items]
Treasury stock issued for shareholder dividend reinvestment	29	27	26
Parent Company
Condensed Cash Flow Statements, Captions [Line Items]
Interest paid	195	209	235
Parent Company | Shareholder dividend reinvestment
Condensed Cash Flow Statements, Captions [Line Items]
Treasury stock issued for shareholder dividend reinvestment	$ 29	$ 26	$ 26