Loss Per Share - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Apr. 22, 2021	Mar. 03, 2021	Jun. 15, 2020
Class of Stock [Line Items]
Warrants recorded within stockholders' equity	$ 52		$ 57	$ 99	$ 57
Proceeds from the issuance of common stock				$ 500
PSP3
Class of Stock [Line Items]
Number of securities called by warrants (in shares)						1,500,000
2020 ATM Offering
Class of Stock [Line Items]
Number of shares issuable (in shares)								28,000,000
Number of shares issued (in shares)		7,000,000
Sale price (in dollars per share)		$ 42.98
Proceeds from the issuance of common stock		$ 282
2021 ATM Offering
Class of Stock [Line Items]
Number of shares issuable (in shares)							37,000,000
Number of shares issued (in shares)				4,000,000
Sale price (in dollars per share)	$ 57.50			$ 57.50
Proceeds from the issuance of common stock				$ 250
Payroll Support Program 2 (PSP2) Warrants
Class of Stock [Line Items]
Warrants to purchase securities (in shares)		2,000,000.0
Warrant exercise price (in dollars per share)	43.26	$ 43.26		$ 43.26
Warrant term		5 years
Warrants recorded within stockholders' equity		$ 56
PSP3 Warrants
Class of Stock [Line Items]
Warrant exercise price (in dollars per share)	$ 53.92			$ 53.92
Warrant term	5 years			5 years
Warrants recorded within stockholders' equity	$ 43