Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jan. 01, 2018	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue	$ 11,953	$ 11,061	$ 33,696	$ 30,910
Operating expense	10,311	9,238	29,535	26,105
Non-operating expense	(32)	47	$ 397	348
Air France-KLM
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restriction period on sale or transfer of shares (in years)			5 years
Passenger
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue	10,796	9,979	$ 30,107	27,925
Frequent Flyer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue (current and noncurrent)	6,542	6,175	6,542	6,175	$ 6,321		$ 5,922
Deferred revenue, current	2,935		2,935		2,762
Air Traffic
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue	9,553	8,812	26,514	24,523
Deferred revenue, current	5,535		5,535		4,364
Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue	$ 931	891	2,938	2,443
ASU No. 2014-09 | Passenger | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue					2,000
ASU No. 2014-09 | Frequent Flyer | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue (current and noncurrent)					2,200
ASU No. 2014-09 | Air Traffic | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue, current					(524)
ASU No. 2014-09 | Other | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue					$ (2,000)
ASU No. 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of adoption of new accounting standard on retained earnings						$ 162
ASU No. 2016-01 | Air France-KLM
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized gain on investment			$ 148
ASU No. 2017-07 | Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating expense		(12)		(36)
Non-operating expense		$ 12		$ 36