Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Mar. 31, 2015	Jun. 30, 2014	Sep. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsidiary, Sale of Stock [Line Items]
Proceeds from shares sold				$ 204,275	$ 107,895	$ 170,590
Common Stock, Shares Authorized				600,000,000	300,000,000
Offering costs				$ 11,900	$ 7,100	$ 6,100
Public Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range	$ 7.25	$ 4.00	$ 3.14
Shares sold	27,758,620	28,750,000	31,846,950
Common stock issued upon exercise in full over-allotment granted	3,620,689	3,750,000	4,153,950
Proceeds from shares sold	$ 190,000	$ 108,000	$ 95,000
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		12,600,000
Proceeds from shares sold				$ 14,600		$ 34,000
Minimum [Member] | 2012 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range						$ 2.06
Maximum [Member] | 2012 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range						$ 3.38
Common stock authorized for issuance under prior shelf registration statement with SEC, dollar amount				$ 50,000