Financial Derivative Instruments - Premiums for Fuel Derivative Contracts (Details) - Fuel derivatives - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Designated as Hedging Instrument | Operating Expense [Member]
Derivative [Line Items]
Premiums paid for fuel derivative contracts	$ 13	$ 28	$ 38	$ 55
Not Designated as Hedging Instrument | Other (gains)/losses, net
Derivative [Line Items]
Premiums paid for fuel derivative contracts	$ 11	$ 0	$ 11	$ 0