Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018
Aflac Japan
Significant Accounting Policies [Line Items]
Percentage of the Company's total assets	84.00%			84.00%
Percentage of the Company's total revenues	68.00%	71.00%
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle, cumulative effect of change on assets			$ 134
New accounting pronouncement or change in accounting principle, cumulative effect of change on liabilities			$ 134