Warrant Transaction Agreements - Walmart Stores, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Millions				12 Months Ended
	Nov. 02, 2020 $ / shares	Jan. 01, 2019 $ / shares	Jul. 20, 2017 USD ($) installment $ / shares shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Jul. 31, 2017 shares
Warrant Transaction Agreements
Reduction in revenue | $			$ 10.9		$ 399.7
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
Warrants issued with the Walmart Stores, Inc transaction agreement
Warrant Transaction Agreements
Shares of common stock that can be purchased from warrants issued (in shares)			55,286,696	20,368,782			55,286,696
Reduction in revenue | $				$ 6.1	$ 5.0	$ 2.4
Cash payments to be received under agreement | $			$ 200.0
Warrant shares vested (in shares)					13,094,217
Number of warrants exercised (in shares)				13,094,217	5,819,652
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Warrant Transaction Agreements
Warrant shares vested (in shares)			5,819,652
Exercise price of warrants (in dollars per share) | $ / shares			$ 2.1231
Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Cash payments to be received under agreement | $			$ 50.0
Warrant shares vested (in shares)			29,098,260
Number of installments | installment			4
Number of shares per installment			7,274,565
Tranche three of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Shares of common stock that can be purchased from warrants issued (in shares)			20,368,784
Cash payments to be received under agreement | $			$ 50.0
Number of installments | installment			8
Number of shares per installment			2,546,098
Amazon
Warrant Transaction Agreements
Risk-free interest rate	0.58%				0.58%
Volatility	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
Stock price | $ / shares	$ 15.47				$ 33.91
Walmart
Warrant Transaction Agreements
Risk-free interest rate		2.63%
Volatility		95.00%
Expected average term		8 years 6 months 18 days
Exercise price | $ / shares		$ 2.12
Stock price | $ / shares		$ 1.24
Walmart | Warrants issued with the Walmart Stores, Inc transaction agreement
Warrant Transaction Agreements
Number of warrants exercised (in shares)				13,094,217	5,819,652
Maximum | Warrants issued with the Walmart Stores, Inc transaction agreement
Warrant Transaction Agreements
Cash payments to be received under agreement | $			$ 600.0
Maximum | Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Cash payments to be received under agreement | $			200.0
Maximum | Tranche three of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Cash payments to be received under agreement | $			$ 400.0