Comprehensive Income (Loss) - Reclassifications (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Fuel and oil	$ 803	$ 257	$ 1,272	$ 1,128
Other (gains) losses, net	(14)	32	(61)	60
Less: Tax expense	154	(324)	185	(374)
Other operating expenses	586	220	1,049	917
Interest expense	116	96	229	124
Net income (loss)	348	$ (915)	463	$ (1,009)
Reclassification to earnings
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net income (loss)	10		(19)
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 derivatives | Fuel and Interest Rate Derivatives | Reclassification to earnings
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Fuel and oil	12		28
Other (gains) losses, net			6
Less: Tax expense	3		8
Net income (loss)	9		26
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		0
Other operating expenses	1
Interest expense			2
Net income (loss)	$ 1		$ 2