Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Oct. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Caribou [Member]
Operating Leased Assets [Line Items]
Percentage of shares owned			10.70%
Caribou [Member] | Maximum [Member] | Development And Regulatory Milestones [Member]
Operating Leased Assets [Line Items]
Milestone payments			$ 6,400,000
Caribou [Member] | Maximum [Member] | Sales Based Milestones [Member]
Operating Leased Assets [Line Items]
Milestone payments			20,000,000
Property Leases [Member]
Operating Leased Assets [Line Items]
Rent expense			6,000,000	$ 2,700,000	$ 600,000
Property Leases [Member] | Cambridge, Massachusetts [Member]
Operating Leased Assets [Line Items]
Operating lease security deposit	$ 2,200,000	$ 300,000
Capital lease obligation additional period of term of leases in years	3 years	5 years
Operating lease arrangement term	10 years
Number of lease years before optional extension and early termination can be exercised	6 years
Prepaid lease payments			$ 3,300,000