Schedule II - Aflac Incorporated (Parent Only) - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Cash flows from operating activities:
Net earnings		$ 4,604	$ 2,659		$ 2,533
Adjustments to reconcile net earnings to net cash provided from operating activities:
Other, net		150	340	[1]	955	[1]
Net cash provided (used) by operating activities		6,128	5,987		6,776
Cash flows from investing activities:
Fixed maturity securities sold		3,819	5,157		3,224
Fixed maturity securities purchased		(9,867)	(10,890)		(6,507)
Other investments sold (purchased)		(2,018)	(1,118)		(70)
Settlement of derivatives		(621)	1,252		(2,119)
Payments for (Proceeds from) Other Investing Activities		(68)	(76)		(86)
Net cash provided (used) by investing activities		(5,431)	(3,855)		(4,897)
Cash flows from financing activities:
Purchases of treasury stock		(1,351)	(1,422)		(1,315)
Proceeds from borrowings		1,040	986		998
Principal payments under debt obligations		(1,161)	(610)		(1,272)
Dividends paid to shareholders		(661)	(658)		(656)
Treasury stock reissued		33	46		36
Proceeds from exercise of stock options		58	68		68
Other, net		0	(120)	[1]	(234)	[1]
Net cash provided (used) by financing activities		(2,065)	(1,619)		(2,187)
Net change in cash and cash equivalents		(1,368)	509		(308)
Cash and cash equivalents, beginning of period		4,859	4,350		4,658
Cash and cash equivalents, end of period		3,491	4,859		4,350
Parent Company
Cash flows from operating activities:
Net earnings		4,604	2,659		2,533
Adjustments to reconcile net earnings to net cash provided from operating activities:
Equity in earnings of subsidiaries	[2],[3]	(4,627)	(2,834)		(2,684)
Cash dividends received from subsidiaries		2,001	2,020		2,393
Other, net		(46)	294	[4]	155	[4]
Net cash provided (used) by operating activities		1,932	2,139		2,397
Cash flows from investing activities:
Fixed maturity securities sold		263	225		121
Fixed maturity securities purchased		(329)	(229)		(202)
Other investments sold (purchased)		(47)	6		14
Settlement of derivatives		223	0		147
Additional capitalization of subsidiaries	[3]	(69)	(36)		(43)
Payments for (Proceeds from) Other Investing Activities		(218)	(25)		0
Net cash provided (used) by investing activities		(177)	(59)		37
Cash flows from financing activities:
Purchases of treasury stock		(1,351)	(1,422)		(1,315)
Proceeds from borrowings		1,040	986		998
Principal payments under debt obligations		(1,161)	(621)		(1,272)
Dividends paid to shareholders		(661)	(658)		(656)
Treasury stock reissued		33	46		36
Proceeds from exercise of stock options		38	36		47
Net change in amount due to/from subsidiary	[3]	(5)	(6)		43
Other, net		0	(125)	[4]	(232)	[4]
Net cash provided (used) by financing activities		(2,067)	(1,764)		(2,351)
Net change in cash and cash equivalents		(312)	316		83
Cash and cash equivalents, beginning of period		2,037	1,721		1,638
Cash and cash equivalents, end of period		$ 1,725	2,037		1,721
Senior Notes due 2039 and 2040
Condensed Cash Flow Statements, Captions [Line Items]
Expense on extinguishment of debt			$ 137
8.50% senior notes due May 2019
Condensed Cash Flow Statements, Captions [Line Items]
Expense on extinguishment of debt					$ 230

[1] Operating activities excludes and financing activities includes a cash outflow of $137 in 2016 and $230 in 2015 for the payments associated with the early extinguishment of debt

[2] Eliminated in consolidation

[3] Eliminated in consolidation

[4] Operating activities excludes and financing activities includes a cash outflow of $137 in 2016 and $230 in 2015 for the payments associated with the early extinguishment of debt