Warrant Transaction Agreements - Amazon.com, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Thousands						3 Months Ended								6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares shares	Nov. 02, 2020 USD ($) installment $ / shares shares	Jan. 01, 2019 USD ($) installment shares	Jul. 20, 2017 USD ($)	Apr. 04, 2017 USD ($) shares	Dec. 31, 2020 $ / shares shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Jul. 31, 2017 shares	Apr. 30, 2017 shares
Class of Warrant or Right [Line Items]
Reduction in revenue				$ 10,900														$ 399,700
Selling, general and administrative							$ 17,210	$ 21,644	$ 11,109	$ 9,973	$ 10,395	$ 13,613	$ 9,221	$ 32,753	$ 22,834	$ 49,963	$ 33,229	$ 79,348	$ 43,202	$ 37,685
Exercise price calculation				The exercise price of the third tranche of Walmart Warrant Shares will be an amount per share equal to ninety percent (90%) of the 30-day volume weighted average share price of the common stock as of the final vesting date of the second tranche of Walmart Warrant Shares
Risk-free interest rate	0.58%	0.58%				0.58%												0.58%
Volatility	75.00%	75.00%				75.00%												75.00%
Expected average term	6 years 3 months 3 days	6 years 5 months 1 day
Exercise price | $ / shares	$ 13.81	$ 13.81				$ 13.81												$ 13.81
Stock price | $ / shares	$ 33.91	15.47				$ 33.91												$ 33.91
Warrant Transaction Agreements
Class of Warrant or Right [Line Items]
Shares of common stock that can be purchased from warrants issued (in shares) | shares					55,286,696																55,286,696	55,286,696
Reduction in revenue	$ 399,700																	$ 420,000	$ 4,100	9,800
Warrant shares vested (in shares) | shares	55,286,696					55,286,696				20,368,782								55,286,696	20,368,782
Fair value of warrants per share | $ / shares	$ 26.95	$ 10.57
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Class of Warrant or Right [Line Items]
Shares of common stock that can be purchased from warrants issued (in shares) | shares					5,819,652
Cash payments to be received under agreement					$ 600,000
Selling, general and administrative					$ 6,700
Exercise price calculation						$1.1893
Tranche two of warrants issued with the Amazon.com, Inc. Transaction Agreement
Class of Warrant or Right [Line Items]
Cash payments to be received under agreement			$ 50,000
Reduction in revenue																		$ 9,000	$ 4,100	$ 9,800
Warrant shares vested (in shares) | shares			29,098,260
Number of installments | installment			4
Tranche two of warrants issued with the Amazon.com, Inc. Transaction Agreement | Maximum
Class of Warrant or Right [Line Items]
Cash payments to be received under agreement			$ 200,000
Tranche three of warrants issued with the Amazon.com, Inc. Transaction Agreement
Class of Warrant or Right [Line Items]
Cash payments to be received under agreement		$ 50,000
Reduction in revenue																		$ 24,100
Warrant shares vested (in shares) | shares		20,368,784
Number of installments | installment		8
Exercise price of warrants (in dollars per share) | $ / shares	$ 13.81					$ 13.81												$ 13.81
Fair value of warrants per share | $ / shares		$ 10.57
Tranche three of warrants issued with the Amazon.com, Inc. Transaction Agreement | Maximum
Class of Warrant or Right [Line Items]
Cash payments to be received under agreement		$ 400,000
Warrant Issued With Amazon
Class of Warrant or Right [Line Items]
Reduction in revenue																		$ 399,700
Warrant shares vested (in shares) | shares	5,354,905	12,730,490				5,354,905												5,354,905