Summary of Significant Accounting Policies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Aflac Japan
Significant Accounting Policies [Line Items]
Aflac Japan's percentage of the Company's total revenues	69.00%	74.00%
Percentage of the Company Assets	83.00%		83.00%
Upper Limit
Significant Accounting Policies [Line Items]
Estimation of Percent of Leases Within Scope of the Guidance to Total Assets	1.00%
Estimation of Percent of Revenues Within Scope of the Guidance to Total Revenues	1.00%