Warrant Transaction Agreements - Amazon.com, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
	Apr. 04, 2017 USD ($) installment $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Jul. 20, 2017 $ / shares	Apr. 30, 2017 shares
Class of Warrant or Right [Line Items]
Selling, general and administrative | $			$ 44,333	$ 38,198	$ 45,010
Exercise price of warrants (in dollars per share) | $ / shares						$ 2.1231
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						55,286,696
Cash payments to be received under agreement | $	$ 600,000
Warrant shares vested (in shares) | shares			20,368,782	20,368,782
Aggregate cash receipts for all installments | $	$ 200,000
Provision for common stock warrants | $			$ 4,100	$ 9,800
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			$ 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
Warrant shares vested (in shares) | shares	29,098,260
Number of installments | installment	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
Warrant shares vested (in shares) | shares	20,368,784
Number of installments | installment	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