Warrant Transaction Agreements - Walmart Stores, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Millions		3 Months Ended		12 Months Ended
	Jul. 20, 2017 USD ($) item $ / shares shares	Mar. 31, 2019 USD ($) shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 shares
Warrant Transaction Agreements
Exercise price of warrants (in dollars per share) | $ / shares	$ 2.1231
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
Warrant shares vested (in shares) | shares		5,819,652			5,819,652
Provision for common stock warrants | $		$ 3.0	$ 0.2
Warrants issued with the Walmart Stores, Inc transaction agreement | Maximum
Warrant Transaction Agreements
Cash payments to be received under agreement | $	$ 600.0
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | shares	5,819,652
Provision for common stock warrants | $				$ 10.9
Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | shares	29,098,260
Number of installments | item	4
Number of shares per installment | shares	7,274,565
Cash receipt per installment | $	$ 50.0
Aggregate cash receipts for all installments | $	$ 200.0
Tranche three of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | shares	20,368,784
Number of installments | item	8
Number of shares per installment | shares	2,546,098
Cash receipt per installment | $	$ 50.0
Aggregate cash receipts for all installments | $	$ 400.0
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
Exercise price of warrants (in dollars per share) | $ / shares	$ 1.1893