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 ($) shares	Dec. 31, 2018 USD ($)
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, provided that, with limited exceptions, the exercise price for the third tranche will be no lower than $1.1893.
Provision for loss contracts related to service									$ 128				$ 142	$ (277)	$ 873	$ (25,110)	$ 1,366	$ (33,125)	1,643	(5,345)
Amazon
Class of Warrant or Right [Line Items]
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 5 months 1 day																6 years 3 months 3 days
Exercise price | $ / shares	$ 13.81	$ 13.81				$ 13.81												$ 13.81
Stock price | $ / shares	$ 33.91	15.47				$ 33.91												$ 33.91
Warrants issued with the Amazon, Inc transaction agreement
Class of Warrant or Right [Line Items]
Shares of common stock that can be purchased from warrants issued (in shares) | shares					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
Exercise price of warrants (in dollars per share) | $ / shares	$ 13.81					$ 13.81												$ 13.81
Fair value of warrants per share | $ / shares	$ 26.95	$ 10.57
Warrant percentage weighted average share price																		90.00%
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
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
Provision for loss contracts related to service																		$ 12,800