Warrant Transaction Agreements - Walmart Stores, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Millions		12 Months Ended
	Jul. 20, 2017 USD ($) installment $ / shares shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)
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
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) | shares	55,286,696
Reduction in revenue		$ 5.0	$ 2.4	$ 0.4
Cash payments to be received under agreement	$ 200.0
Warrant shares vested (in shares) | shares		5,819,652	13,094,217
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | 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) | shares	29,098,260
Number of installments | installment	4
Number of shares per installment | shares	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) | shares	20,368,784
Cash payments to be received under agreement	$ 50.0
Number of installments | installment	8
Number of shares per installment | shares	2,546,098
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