Warrant Transaction Agreements - Walmart Stores, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 20, 2017 USD ($) item $ / shares shares	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) shares
Class of Warrant or Right [Line Items]
Warrant shares vested (in shares) | shares		5,819,652		5,819,652		5,819,652
Selling, general and administrative		$ 12,241	$ 17,904	$ 20,550	$ 27,049
Exercise price of warrants (in dollars per share) | $ / shares	$ 2.1231
Provision for common stock warrants		3,921	$ 1,820	5,806	$ 1,820
Warrants issued with the Walmart Stores, 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
Cash payments to be received under agreement	$ 600,000
Provision for common stock warrants		$ 300		$ 500
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Class of Warrant or Right [Line Items]
Warrant shares vested (in shares) | shares	5,819,652
Provision for common stock warrants						$ 10,900
Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Class of Warrant or Right [Line Items]
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	$ 50,000
Aggregate cash receipts for all installments	$ 200,000
Tranche three of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Class of Warrant or Right [Line Items]
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	$ 50,000
Aggregate cash receipts for all installments	$ 400,000
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