Revenue Arrangements - Spark license agreement (Narrative) (Details)		1 Months Ended	3 Months Ended
	Dec. 02, 2016 USD ($) target	Dec. 31, 2016 USD ($) $ / shares shares	Mar. 31, 2017 USD ($)
Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 10,000,000.0
Aggregate additional payments due within twelve months	$ 5,000,000.0
Number of additional targets | target	4
Variable fee the Company is eligible to receive for each additional target option elected	$ 2,000,000.0
Election period to exercise additional targets	3 years
Additional development and regulatory milestone payments	$ 65,000,000.0
Commercialization milestone payments	$ 365,000,000.0
Period after first commercial sale when the Company is eligible to receive royalties	10 years
Period for prior written notice to terminate license	90 days
Number of shares issued | shares		197,238
Gross proceeds		$ 5,000,000
Share price (in dollars per share) | $ / shares		$ 25.35
Premium of average daily VWAP (as a percent)		115.00%
Consecutive calendar days leading up to and ending on the day prior to the Contract Date		30 days
Number of shares authorized to be issued pursuant to the Stock Purchase Agreement | shares		2,758,112
First Acquisition Right
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate price of shares Spark will have the right to purchase		$ 5,000,000.0
Second Acquisition Right
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregate price of shares Spark will have the right to purchase		$ 5,000,000.0
MIT | Spark License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contractual payments made for MIT sub-license provided to Spark	$ 1,500,000		$ 1,500,000
Payment relative to calculated premium paid by Spark for initial equity investment	$ 200,000		$ 200,000