Stock-Based Compensation​ - Narrative (Details)										1 Months Ended			3 Months Ended				9 Months Ended
	Aug. 10, 2023 USD ($) shares	Jun. 23, 2023 USD ($) $ / shares shares	Jan. 03, 2023 USD ($) $ / shares shares	Jan. 01, 2023 shares	Aug. 09, 2022 aircraft $ / shares shares	Apr. 14, 2022 shares	Apr. 13, 2022 tranche shares	Sep. 15, 2021 shares	Jan. 29, 2021 milestone aircraft installment $ / shares shares	Aug. 31, 2023 USD ($) shares	Jun. 30, 2022	Aug. 31, 2021 shares	Sep. 30, 2023 USD ($) tranche $ / shares shares	Jun. 30, 2023 shares	Sep. 30, 2022 USD ($) tranche shares	Jun. 30, 2022 shares	Sep. 30, 2023 USD ($) tranche $ / shares shares	Sep. 30, 2022 USD ($) tranche shares	Jul. 13, 2023 USD ($) tranche shares	Jun. 01, 2023 $ / shares	Dec. 31, 2022 shares	Apr. 30, 2022 shares	Sep. 16, 2021 tranche
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													$ (27,400,000)		$ 26,200,000		$ 24,800,000	$ 76,300,000
Amount reclassified from legal expense to stock-based compensation expense | $										$ 11,000,000
Number of conditional purchased aircraft | aircraft									200
Option to purchase of additional aircraft | aircraft									100
Other warrant expense | $													0		6,000,000.0		2,100,000	6,000,000
Research and development | $													$ 67,800,000		$ 47,000,000.0		196,900,000	112,300,000
Founder grants, vesting percentage upon achievements								25.00%
Proceeds from issuance of common stock | $																	25,000,000.0	$ 0
Recognized gain | $																	$ 3,600,000
United
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cashless exercise of warrants (in shares) | $										2,948,352
Sub Milestone One and Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of each warrant (in dollars per share) | $ / shares					$ 4.37
United Airlines Inc.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of conditional purchased aircraft | aircraft					100
Stellantis N.V.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants price per share (in dollars per share) | $ / shares			$ 0.01
Warrants, weighted average grant date fair value (in dollars per share) | $ / shares			$ 1.93
Stellantis N.V. | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount applicable to milestone | $	$ 70,000,000
Shares issuable (in shares)	12,313,234
Vendor
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accounts payable | $										$ 11,000,000
Warrants for Collaboration Agreement with United Airlines Inc.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Class of warrant or right, vested during the period (in shares)													2,211,264		737,088		2,211,264	737,088
Warrants for Collaboration Agreement with United Airlines Inc. | United Airlines Inc.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants, issued or issuable (in shares)									14,741,764
Warrants price per share (in dollars per share) | $ / shares									$ 0.01
Number of vesting installments | installment									4
Number of vesting milestones | milestone									4
Amended United Warrant Agreement | United Airlines Inc. | Sub Milestone One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrant vested and expected to vest (in shares)					737,088
Amended United Warrant Agreement | United Airlines Inc. | Sub Milestone Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrant vested and expected to vest (in shares)					2,211,264
Amended United Warrant Agreement | United Airlines Inc. | Sub Milestone Three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of conditional purchased aircraft | aircraft					160
Warrant vested and expected to vest (in shares)					3,685.45
Amended United Warrant Agreement | United Airlines Inc. | Sub Milestone Four
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of conditional purchased aircraft | aircraft					40
Warrant vested and expected to vest (in shares)					22,112.65
Warrants for Collaboration Agreement with FCA US LLC | Stellantis N.V.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Research and development | $													$ 4,400,000				$ 13,100,000
Warrant Exercisable Period One | Stellantis N.V.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)			12 months
Warrant Exercisable Period Two | Stellantis N.V.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)			24 months
Warrant Exercisable Period Three | Stellantis N.V.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)			36 months
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													(34,900,000)		$ 19,500,000		3,900,000	$ 57,300,000
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													7,500,000		6,700,000		20,900,000	19,000,000.0
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of outstanding stock maximum												1.00%
Purchase price of common stock, percent												85.00%
Number of additional shares issuable (in shares)												9,938,118
Stock based compensation expense | $													400,000		0		1,000,000	0
Unvested stock options | $													200,000				200,000
Share price (in dollars per share) | $ / shares																				$ 2.99
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													$ 7,700,000		5,400,000		$ 19,200,000	16,500,000
Weighted-average period																	1 year 2 months 12 days
Granted (in shares)								20,009,224									10,879,413
Holders ownership once grants are fully vested								18.00%
Milestone achievement period (in years)								7 years
Founder grants, number of tranches | tranche																							4
Vested (in shares)																	5,704,993
Forfeited (in shares)																	15,847,964
Number of outstanding (in shares)													32,473,088				32,473,088				43,146,632
Share-based payment arrangement, nonvested award, cost not yet recognized, amount | $													$ 139,300,000				$ 139,300,000
Restricted Stock Units | General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													$ 8,200,000		$ 16,300,000		$ 24,300,000	$ 48,500,000
Remaining number of tranches | tranche													3		3		3	3
Restricted Stock Units | General and administrative | Reversed in July 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $															$ 8,200,000			$ 24,300,000
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													$ 600,000		900,000		$ 2,000,000	3,000,000
Unvested stock options | $													$ 5,800,000				$ 5,800,000
Weighted-average period																	9 months 18 days
Amended and Restated 2021 Plan | Quarterly Equity Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion ratio																	1
Amended and Restated 2021 Plan | Subject to Forfeiture if Vesting Conditions are not Met
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion ratio																	1
Amended and Restated 2021 Plan | Tranche One | Subject to Forfeiture if Vesting Conditions are not Met
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)																	1 year
Amended and Restated 2021 Plan | Restricted Stock Units | Quarterly Equity Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)																	998,364
Amended and Restated 2021 Plan | Restricted Stock Units | Subject to Forfeiture if Vesting Conditions are not Met
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)																	9,881,049
Amended and Restated 2021 Plan | Restricted Stock Units | Minimum | Subject to Forfeiture if Vesting Conditions are not Met
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)																	3 years
Amended and Restated 2021 Plan | Restricted Stock Units | Maximum | Subject to Forfeiture if Vesting Conditions are not Met
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)																	4 years
2021 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employee purchased shares (in shares)																	601,105
Employee purchased per share (in dollars per share) | $ / shares													$ 2.19				$ 2.19
Quarterly Equity Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense | $													$ 2,800,000		$ 2,600,000		$ 8,800,000	$ 6,600,000
Common Class B | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited (in shares)																5,002,306
Common Class B | Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)							15 months
Vested (in shares)								5,002,306
Forfeited (in shares)						5,002,306
Number of outstanding (in shares)							15,006,918												15,006,918
Remaining number of tranches | tranche							3												3
Reversal of expense | $																			$ 59,100,000
Shares outstanding (in shares)													15,006,918				15,006,918
Common Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Contributions up to (as percent)																	15.00%
Common Class A | Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise of warrants (in shares)													2,942,778
Issuance of common stock (in shares)										1,985,559			1,985,559	6,337,039
Common Class A | Stellantis N.V.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Sale of stock value | $			$ 150,000,000
Number of securities called by warrants or rights (in shares)			15,000,000
Share price (in dollars per share) | $ / shares			$ 1.93
Issuance of common stock (in shares)		6,337,039
Price per share (in dollars per share) | $ / shares		$ 3.94506
Proceeds from issuance of common stock | $		$ 25,000,000
Price per share (in dollars per share) | $ / shares		3.38
Common Class A | Warrants for Collaboration Agreement with United Airlines Inc. | United Airlines Inc.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of securities called by each warrant or right (in shares)									1
Common Class A | 2021 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capital shares reserved for future issuance (in shares)				12,292,155																		34,175,708
Percentage of outstanding stock maximum											5.00%
Common Class A | 2021 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)				1,809,383
Maximum number of shares authorized for issuance (in share)													8,406,337				8,406,337
Number of shares remained available (in shares)													7,805,232				7,805,232