Stockholders' Equity - Warrants Outstanding by Expiration Date (Details) shares in Thousands	Dec. 31, 2018 $ / shares shares
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	437
Warrants Expiring March 2020, Tranche 2
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	13
Exercise price (in USD per share) | $ / shares	$ 18.00
Warrants Expiring April 2022
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	8
Exercise price (in USD per share) | $ / shares	$ 52.50
Warrants Expiring May 2022
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	82
Exercise price (in USD per share) | $ / shares	$ 46.50
Warrants Expiring January 2023
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	11
Exercise price (in USD per share) | $ / shares	$ 12.60
Warrants Expiring January 2023, Tranche 2
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	3
Exercise price (in USD per share) | $ / shares	$ 21.30
Warrants Expiring March 2023
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	11
Exercise price (in USD per share) | $ / shares	$ 12.60
Warrants Expiring March 2023, Tranche 2
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	4
Exercise price (in USD per share) | $ / shares	$ 21.30
Warrants Expiring March 2023, Tranche 3
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	12
Exercise price (in USD per share) | $ / shares	$ 18.90
Warrants Expiring May 2023
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	80
Exercise price (in USD per share) | $ / shares	$ 15.00
Warrants Expiring March 2024
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	8
Exercise price (in USD per share) | $ / shares	$ 10.14
Warrants Expiring March 2024 Tranche 2
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	115
Exercise price (in USD per share) | $ / shares	$ 9.00
Warrants Expiring March 2024 Tranche 3
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	89
Exercise price (in USD per share) | $ / shares	$ 12.00
Warrants Expiring October 2024
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	1
Exercise price (in USD per share) | $ / shares	$ 387.00
Warrants Expiring July 2025
Class of Warrant or Right [Line Items]
Shares issuable upon exercise (in shares)	1
Exercise price (in USD per share) | $ / shares	$ 492.00