Accrued Expenses and Other Short Term Liabilities (Narrative) (Details)		1 Months Ended		3 Months Ended	7 Months Ended	12 Months Ended
	Aug. 01, 2016 shares	Mar. 31, 2017 USD ($) shares	Jun. 30, 2016 $ / shares shares	Sep. 30, 2016 USD ($)	Jul. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / shares item shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Sep. 30, 2015 USD ($) $ / shares	Oct. 31, 2014 $ / shares
Short-Term Accrued Expenses [Line Items]
Payments of initial public offering costs							$ 3,293,000	$ 445,000
Stock issued, shares | shares	6,250,000
Severance expense						$ 2,900,000	2,900,000
Severance cost						1,122,000	1,744,000
Accrued expenses						8,577,000	12,150,000
Other long term liabilities						$ 247,000	$ 1,250,000
Units granted | shares						13,394,338	617,651
Term of lock-up agreement related to IPO						180 days
Percentage ownership triggering reduced lock-up term						5.00%
Term of maximum lock-up agreement in event of 5% ownership and subsequent offering (90 days)						90 days
Royalty arrangements							$ 2,502,000
Deferred revenue, short term						$ 4,400,000	4,400,000
Royalty Arrangement [Member]
Short-Term Accrued Expenses [Line Items]
Royalty expense						100,000	1,200,000	$ 2,700,000
Deferred revenue, short term							2,200,000
Board of Directors Chairman [Member]
Short-Term Accrued Expenses [Line Items]
Severance expense						1,200,000	2,200,000
Severance cost						3,000,000
Severance costs, due in year one						1,000,000	900,000
Severance costs, due in year two						1,100,000
Severance costs including medical benefits						3,100,000
Accrued expenses						1,000,000	1,000,000
Other long term liabilities						$ 200,000	$ 1,200,000
Percent of cash received or receivable from business development programs to be used for research and development						5.00%
Cap on cash received or receivable from business development programs to be used for research and development						$ 15,000,000
Number of business development programs qualifying for contributions to research and development | item						3
Cash received or receivable from business development program before IPO to trigger fulfillment of conditional severance payments						$ 800,000,000
Board of Directors Chairman [Member] | Supplemental Conditional Payments [Member]
Short-Term Accrued Expenses [Line Items]
Aggregate payments						6,750,000
Payments, 2017						2,250,000
Payments, 2018						2,250,000
Payments, 2019						2,250,000
Minimum payables to trigger supplemental conditional payments						50,000,000
Class A Units [Member]
Short-Term Accrued Expenses [Line Items]
Shares issued for advisory agreement | shares							25,000	1,808,334
Unit price | $ / shares									$ 32.50	$ 39.00
Class E Redeemable Convertible Units [Member]
Short-Term Accrued Expenses [Line Items]
Shares issued for advisory agreement | shares							1,170,437	574,392
Stock issued, shares | shares							478,266
Unit price | $ / shares			$ 11.50
Class E Redeemable Convertible Units [Member] | Board of Directors Chairman [Member]
Short-Term Accrued Expenses [Line Items]
Stock issued, shares | shares			21,740				21,740
2015 Stock Issuance [Member]
Short-Term Accrued Expenses [Line Items]
Accrued liabilities current, commissions payable						40,000	$ 40,000
2014 Stock Issuance [Member]
Short-Term Accrued Expenses [Line Items]
Accrued liabilities current, commissions payable						2,400,000	2,400,000
2017 Private Placement [Member]
Short-Term Accrued Expenses [Line Items]
Payments of initial public offering costs		$ 1,800,000				$ 1,800,000
Stock issued, shares | shares		6,767,855				6,767,855
2015 Second-Lien Convertible Debt [Member]
Short-Term Accrued Expenses [Line Items]
Borrowings, face amount							15,000,000		$ 92,000,000
Equity Appreciation Rights Units [Member] | Board of Directors Chairman [Member]
Short-Term Accrued Expenses [Line Items]
Price of units granted | $ / shares						$ 6.00
2014 Long-Term Incentive Plan (“LTIP”) [Member]
Short-Term Accrued Expenses [Line Items]
Stock compensation expense				$ 22,600,000	$ 0
2014 Long-Term Incentive Plan (“LTIP”) [Member] | Board of Directors Chairman [Member]
Short-Term Accrued Expenses [Line Items]
Stock compensation expense				$ 11,600,000
2014 Long-Term Incentive Plan (“LTIP”) [Member] | Equity Appreciation Rights Units [Member] | Board of Directors Chairman [Member]
Short-Term Accrued Expenses [Line Items]
Vesting condition, Percent increase in fair market value per unit						333.00%
Award adjustment percentage						0.75%
Shares authorized for issuance | shares						1,783,618
Third Party Investors [Member]
Short-Term Accrued Expenses [Line Items]
Gross proceeds from issuance of redeemable convertible units							873,000	$ 39,500,000
Net proceeds from issuance of redeemable convertible units							833,000	36,400,000
Payments of initial public offering costs							$ 40,000	$ 3,100,000
Third Party Investors [Member] | Class E Redeemable Convertible Units [Member]
Short-Term Accrued Expenses [Line Items]
Stock issued, shares | shares							75,875	3,438,984