Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions			3 Months Ended
		Feb. 13, 2018	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Stock Split, Conversion Ratio		2
Common stock as a percentage of two-for-one stock split dividend		100.00%
Aflac Japan
Significant Accounting Policies [Line Items]
Percentage of the Company's total assets			85.00%		83.00%
Percentage of the Company's total revenues			71.00%	69.00%
Upper Limit | Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Estimation of Percent of Leases Within Scope of the Guidance to Total Assets			1.00%
Accumulated other comprehensive income (loss): | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes	[1]		$ (148)	$ 0
Accumulated other comprehensive income (loss): | Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes	[1]		374	0
Retained earnings | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes	[1]		148	0
Retained earnings | Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes	[1]		$ (374)	$ 0

[1] See Note 1 of the Notes to the Consolidated Financial Statements for the adoption of accounting guidance on January 1, 2018.