Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2012	May. 31, 2016
Subsidiary, Sale of Stock [Line Items]
Sales per share price range		$ 5.56			$ 5.56
Convertible Debt [Member]
Subsidiary, Sale of Stock [Line Items]
Adjustments to Additional Paid In Capital Costs and Expenses of Capped Call Transactions		$ 38.5
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	$ 190.0
Offering costs	$ 11.6
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			$ 14.6
Maximum [Member] | 2012 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock authorized for issuance under prior shelf registration statement with SEC, dollar amount				$ 50.0