Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		2 Months Ended	3 Months Ended	12 Months Ended
	Jan. 17, 2018	Mar. 31, 2015	Mar. 09, 2018	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2015	Jan. 31, 2017	Jan. 25, 2016
Subsidiary, Sale of Stock [Line Items]
Sales per share price range								$ 5.56
Proceeds from shares sold					$ 63,425	$ 204,275
Convertible Debt [Member]
Subsidiary, Sale of Stock [Line Items]
Adjustments to Additional Paid IN capital, Cost and Expenses on Derivative Transaction				$ 38,500
Public Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range		$ 7.25
Shares sold		27,758,620
Common stock issued upon exercise in full over-allotment granted		3,620,689
Proceeds from shares sold		$ 190,000
Offering costs		$ 11,600
2012 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range						$ 10.63
Shares sold						1,400,000
Proceeds from shares sold						$ 14,600
December 2017 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Shelf Registration Statement, Maximum Authorized Common Stock Issuance, Value					$ 75,000
December 2017 Sales Agreement [Member] | Subsequent Event [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold			12,700,000
Proceeds from shares sold			$ 26,000
Shelf Registration Statement, Remaining Authorized Stock Issuance,Value			$ 48,600
January 2017 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range					$ 1.27
Shares sold					50,900,000
Proceeds from shares sold					$ 63,400
Shelf Registration Statement, Maximum Authorized Common Stock Issuance, Value							$ 75,000
January 2017 Sales Agreement [Member] | Subsequent Event [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold	6,800,000
Proceeds from shares sold	$ 10,300