Stockholders' Deficit (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Jan. 31, 2017
Subsidiary, Sale of Stock [Line Items]
Proceeds from shares sold	$ 55,239	$ 42,599
December 2018 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range	$ 0.53
Shares sold	33.6
Proceeds from shares sold	$ 17,400
Shelf Registration Statement, Maximum Authorized Common Stock Issuance, Value				$ 100,000
Shelf Registration Statement Remaining Unissued Capital	$ 82,200
January 2017 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range		$ 1.54
Shares sold		6.8
Proceeds from shares sold		$ 10,300
Shelf Registration Statement, Maximum Authorized Common Stock Issuance, Value					$ 75,000
December 2017 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range	$ 0.77	$ 2.09
Shares sold	50.3	15.7
Proceeds from shares sold	$ 37,900	$ 32,300	$ 75,000