Collaboration and License Agreement - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended			12 Months Ended	15 Months Ended	24 Months Ended	43 Months Ended
	Aug. 31, 2014	Jan. 31, 2017	Dec. 31, 2016	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Mar. 31, 2018	Dec. 31, 2018	Mar. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue				$ 5,331	$ 2,410
Deferred revenue			$ 45,000	28,227	42,590	$ 45,000	$ 33,558	$ 28,227		$ 28,227
Short-term receivable from collaboration partner			45,000			45,000	1,013
Collaborative Agreement | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash received under collaboration agreement	$ 35,000
Up-front equity investment	10,000
Additional equity investments received										14,000
Eligible to receive one-time, non-refundable contingent payment	25,000
Project non-refundable continuation payment	45,000
Milestone payment received	25,000					$ 25,000
Continuation payment, amount received		$ 45,000	$ 45,000
Upfront cash payment	34,300
Net fair value allocated to equity	700
Variable consideration	25,000
Collaboration and license revenue				5,300	2,400
Deferred revenue				28,200				28,200		28,200
Deferred revenue recognized								16,800
Short-term receivable from collaboration partner				0			1,013	$ 0		$ 0
Prepayment from collaboration partner				6,090			$ 4,400
Reduction in research and development expenses due to RPP reimbursements				$ 2,800	$ 0
Collaborative Agreement | Sanofi (Aventis Inc.) | Forecast
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Fixed fee									$ 45,000
Transaction price for extended term									$ 45,000
Collaborative Agreement | Maximum | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Funding from approved in-kind research and clinical activities	45,000
Additional research and development funding for collaboration	$ 15,000