Comprehensive Income (Loss) - Reclassifications (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Fuel and oil	$ 990	$ 379	$ 2,261	$ 1,507
Other (gains) losses, net	29	35	(32)	95
Less: Tax expense	154	(385)	339	(759)
Other operating expenses	662	488	1,710	1,405
Interest expense	115	111	343	235
Net income (loss)	446	$ (1,157)	909	$ (2,166)
Reclassification to earnings
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net income (loss)	2		(16)
Defined benefit plan items | Reclassification to earnings
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Other (gains) losses, net			(60)
Less: Tax expense			(13)
Net income (loss)			(47)
Fuel derivative contracts | Fuel and Interest Rate Derivatives | Reclassification to earnings
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Fuel and oil	2		30
Other (gains) losses, net			6
Less: Tax expense	1		8
Net income (loss)	1		28
Interest rate derivatives | Fuel and Interest Rate Derivatives | Reclassification to earnings
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: Tax expense	0		1
Other operating expenses	1
Interest expense			4
Net income (loss)	$ 1		$ 3