Collaboration and License Agreement - Schedule of Receivables and Prepayments From Collaboration Partner (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivable from collaboration partner, Balance at Beginning of Period	$ 1,013	$ 45,000	$ 45,000
Payments Received	1,013	$ 45,000
Receivable from collaboration partner, Balance at End of Period			1,013
Balance at December 31, 2017	4,432
Balance at March 31, 2018	7,701		4,432
Collaborative Agreement | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivable from collaboration partner, Balance at Beginning of Period	1,013
Payments Received	1,013
Receivable from collaboration partner, Balance at End of Period	0		1,013
Balance at December 31, 2017	4,432
Payments Received	6,090		4,400
Actual Expenses Incurred During the Quarter	(2,821)
Balance at March 31, 2018	$ 7,701		$ 4,432