Warrant Transaction Agreements - Amazon.com, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Apr. 04, 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	Jul. 20, 2017 $ / 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 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	55,286,696
Cash payments to be received under agreement	$ 600,000
Warrant shares vested (in shares) | shares		13,094,217		13,094,217		13,094,217
Provision for common stock warrants		$ 3,600	$ 1,800	$ 5,300	$ 1,800
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Class of Warrant or Right [Line Items]
Warrant shares vested (in shares) | shares	5,819,652
Selling, general and administrative						$ 6,700
Tranche two 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	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
Exercise price of warrants (in dollars per share) | $ / shares	$ 1.1893
Tranche three 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	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 Amazon 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 Amazon Warrant Shares