Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions		9 Months Ended
	Feb. 13, 2018	Sep. 30, 2018	Sep. 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		84.00%				83.00%
Percentage of the Company's total revenues		69.00%	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			$ 0	[1]	$ (148)	$ (148)	[1]
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			0	[1]	374	374	[1]
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.