Collaboration Agreements - Additional Information (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Jan. 17, 2019	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common Stock Shares owned	106,400,000		101,177,000
Sanofi Genzyme | Collaboration Amendment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Investor agreement description	until the earlier of (i) the fifth anniversary of the expiration of the last to expire royalty term or the earlier termination of the collaboration agreement, as amended by the Collaboration Amendment, and (ii) the date after December 31, 2021 on which the beneficial ownership of Sanofi Genzyme and its affiliates no longer represents at least 5% of the outstanding shares of common stock, Sanofi Genzyme and its affiliates will be bound by certain “standstill” provisions, including an agreement not to propose or support a proposal to acquire us.
Sanofi Genzyme | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners	2.50%
Sanofi Genzyme | Minimum | Collaboration Amendment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners	5.00%
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common Stock Shares owned		10,554,134