Financial Derivative Instruments - Location and Amount Recognized in Income by Hedging Relationship (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Cash Flow Hedging
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	$ 186	$ 9	$ 279	$ (107)
Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	192	9	275	(75)
Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	(6)	0	4	(32)
Fuel and oil
Derivative [Line Items]
Derivatives designated as hedging instruments	12	14	28	36
Fuel and oil | Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	12	14	28	36
Fuel and oil | Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	0	0	0	0
Fuel and oil | Fair Value Hedging | Not Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Hedged items	0	0	0	0
Fuel and oil | Fair Value Hedging | Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Derivatives designated as hedging instruments	0	0	0	0
Other (gains)/losses, net
Derivative [Line Items]
Derivatives designated as hedging instruments	0	14	6	16
Other (gains)/losses, net | Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	0	14	6	16
Other (gains)/losses, net | Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	0	0	0	0
Other (gains)/losses, net | Fair Value Hedging | Not Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Hedged items	0	0	0	0
Other (gains)/losses, net | Fair Value Hedging | Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Derivatives designated as hedging instruments	0	0	0	0
Other operating expenses
Derivative [Line Items]
Derivatives designated as hedging instruments	1
Other operating expenses | Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	0
Other operating expenses | Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income	1
Other operating expenses | Fair Value Hedging | Not Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Hedged items	0
Other operating expenses | Fair Value Hedging | Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Derivatives designated as hedging instruments	$ 0
Interest expense
Derivative [Line Items]
Derivatives designated as hedging instruments		3	2	4
Interest expense | Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income		0	0	0
Interest expense | Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Amount of loss reclassified from AOCI into income		1	2	1
Interest expense | Fair Value Hedging | Not Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Hedged items		4	0	7
Interest expense | Fair Value Hedging | Designated as Hedging Instrument | Interest rate derivatives
Derivative [Line Items]
Derivatives designated as hedging instruments		$ (2)	$ 0	$ (4)