Business Overview (Details) - USD ($)							1 Months Ended		3 Months Ended								12 Months Ended
	Nov. 13, 2017	Sep. 01, 2017	Feb. 28, 2017	Feb. 06, 2017	Jan. 31, 2017	Mar. 18, 2016	Mar. 31, 2017	Nov. 30, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 28, 2018	Dec. 31, 2014	Nov. 03, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Cash and cash equivalents									$ 136,986,000				$ 82,387,000				$ 136,986,000	$ 82,387,000	$ 61,653,000		$ 146,470,000
Net cash used in operating activities																	(212,889,000)	(129,753,000)	(100,989,000)
Subsidiary, Sale of Stock [Line Items]
Proceeds of sale of common stock, net of issuance costs																	173,802,000	89,845,000	14,268,000
Payment for indenture consent			$ 1,600,000
Debt conversion amount																	4,912,000	137,941,000	40,989,000
Debt conversion expense									0	$ 0	$ 0	$ 1,209,000	$ 14,543,000	$ 0	$ 0	$ 25,615,000	$ 1,209,000	$ 40,158,000	0
Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Minimum Market Capitalization covenant, amount																				$ 300,000,000
Minimum Market Capitalization covenant, percentage of outstanding principle amount of loan																				200.00%
Common Shares
Subsidiary, Sale of Stock [Line Items]
Shares sold in private placement (in shares)																	42,030,630	29,948,334
Debt conversion per original conversion price (in shares)						9,600,000											212,800	12,161,671
Debt conversion expense (in shares)																	200,000
Convertible Senior Notes Due 2019
Subsidiary, Sale of Stock [Line Items]
Debt conversion amount						$ 79,800,000	$ 4,900,000	$ 55,700,000									$ 4,911,000	$ 135,497,000
Number of shares issued on conversion of debt (in shares)						35,300,000	1,800,000	20,500,000										56,600,000
Debt conversion per original conversion price (in shares)							1,600,000					200,000	2,600,000
Conversion price (in dollars per share)						$ 3.11																$ 3.11
Aggregate principal amount									$ 18,603,000				$ 23,500,000.0				$ 18,603,000	$ 23,500,000.0	$ 159,000,000			$ 200,000,000
Debt conversion expense												$ 1,200,000	$ 14,500,000			$ 25,600,000
Term Loan | CRG Servicing LLC | Secured Debt | Line of Credit
Subsidiary, Sale of Stock [Line Items]
Maximum borrowing capacity (up to)		$ 300,000,000
Proceeds from line of credit		100,000,000
Remaining borrowing capacity		$ 100,000,000
Interest rate per annum		9.50%
Term Loan | CRG Servicing LLC | Secured Debt | Line of Credit | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Maximum borrowing capacity (up to)																				$ 200,000,000
Term Loan | CRG Servicing LLC | Secured Debt | Line of Credit | Tranche One | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Remaining borrowing capacity																				25,000,000
Term Loan | CRG Servicing LLC | Secured Debt | Line of Credit | Tranche Two | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Remaining borrowing capacity																				25,000,000
Term Loan | CRG Servicing LLC | Secured Debt | Line of Credit | Tranche Three | Subsequent Event
Subsidiary, Sale of Stock [Line Items]
Remaining borrowing capacity																				$ 50,000,000
Public stock offering
Subsidiary, Sale of Stock [Line Items]
Shares sold in private placement (in shares)	21,705,426				20,325,204
Shares issued (in dollars per share)	$ 2.58				$ 6.15
Proceeds of sale of common stock, net of issuance costs	$ 52,200,000			$ 121,600,000