Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions			6 Months Ended
		Feb. 13, 2018	Jun. 30, 2018	Jun. 30, 2017 USD ($)		Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
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			70.00%	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]			$ 0		$ (148)
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]			0		374
Retained earnings | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes				0	[1]	148	$ 148	[1]
Retained earnings | Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Cumulative effect of change in accounting principle, net of income taxes				$ 0	[1]	$ (374)	$ (374)	[1]

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