Collaboration and License Agreement - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended	6 Months Ended		12 Months Ended	24 Months Ended	43 Months Ended
	Aug. 31, 2014	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2018	Mar. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue						$ 5,411
Deferred revenue			$ 45,000	$ 45,000	$ 21,588	39,589	$ 33,558
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					12,000	5,400
Deferred revenue					21,600
Deferred revenue recognized					12,000		11,400
Short-term receivable from collaboration partner					0		1,013
Prepayment from collaboration partner					11,809		$ 4,400
Reduction in research and development expenses due to RPP reimbursements					$ 7,100	$ 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