Summary of Significant Accounting Policies - Additional Information (Detail) - segment	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Significant Accounting Policies [Line Items]
Number of reportable insurance business segments	2
Aflac Japan
Significant Accounting Policies [Line Items]
Percentage of the Company's total revenues	70.00%	67.00%
Percentage of the Company's total assets	83.00%		83.00%

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration