Commitments and Contingencies - Additional Information (Detail)					1 Months Ended					12 Months Ended
	Dec. 27, 2017 USD ($)	Apr. 29, 2016 USD ($)	Feb. 28, 2014 ft²	Feb. 10, 2012 ft²	Aug. 31, 2016 RenewalOption	May 31, 2015 USD ($) ft² RenewalOption $ / ft²	Apr. 30, 2015 USD ($) ft² RenewalOption	Mar. 31, 2015 USD ($) RenewalOption	Mar. 31, 2014 RenewalOption	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 14, 2015 USD ($)
Commitments And Contingencies [Line Items]
Fixed, non-cancelable payment due in 2018										$ 5,000,000
Fixed, non-cancelable payment due in 2019										5,000,000
Rent expense										18,700,000	$ 15,900,000	$ 10,500,000
Repayment of outstanding principal amount in full										120,000,000
Restricted investments										30,000,000	150,000,000
Litigation, motion filed for reimbursement of costs and fees													$ 8,000,000
Term Loan Facility | Other Income (Expense)
Commitments And Contingencies [Line Items]
Interest expense										800,000	1,200,000
Bank of America N.A
Commitments And Contingencies [Line Items]
Cash collateral required for principal amount outstanding, percentage		100.00%
Bank of America N.A | Term Loan Facility
Commitments And Contingencies [Line Items]
Line of credit facility, maximum borrowing capacity		$ 120,000,000
Repayment of outstanding principal amount in full	$ 120,000,000
Line of credit facility, expiration date		Apr. 29, 2021
Bank of America N.A | Term Loan Facility | LIBOR
Commitments And Contingencies [Line Items]
Debt instrument, basis spread on variable rate		0.45%
Wells Fargo Bank, National Association
Commitments And Contingencies [Line Items]
Cash collateral required for principal amount outstanding, percentage		100.00%
Wells Fargo Bank, National Association | Term Loan Facility
Commitments And Contingencies [Line Items]
Line of credit facility, maximum borrowing capacity		$ 30,000,000
Line of credit facility, expiration date		Apr. 29, 2021
Wells Fargo Bank, National Association | Term Loan Facility | LIBOR
Commitments And Contingencies [Line Items]
Debt instrument, basis spread on variable rate		0.45%
Other Assets
Commitments And Contingencies [Line Items]
Restricted cash										$ 1,500,000	$ 1,500,000
Third Street Lease
Commitments And Contingencies [Line Items]
Lease extension period									5 years
Lease extended expiration date									Sep. 30, 2021
Area of office and laboratory space held under lease | ft²			129,000
Amended lease extension period									5 years
Number of lease extension options | RenewalOption									1
Lease expiration date			2016-09
BMR-665 Concord Avenue Lease
Commitments And Contingencies [Line Items]
Lease extension period					5 years
Lease extended expiration date					Aug. 31, 2022
Area of office and laboratory space held under lease | ft²				15,000
Amended lease extension period					5 years
Number of lease extension options | RenewalOption					1
Lease expiration date				2017-08
BMR-675 West Kendall Lease
Commitments And Contingencies [Line Items]
Area of office and laboratory space held under lease | ft²							295,000
Number of lease extension options | RenewalOption							2
Lease term							15 years
Lease renewal options period							5 years
Facility lease annual rent for the first year							$ 19,800,000
Facility lease percentage of annual rent increase after first year							3.00%
Cost of base building and tenant improvement							$ 56,100,000
101 Main Street Leases
Commitments And Contingencies [Line Items]
Area of office and laboratory space held under lease | ft²						72,000
Operating lease annual rent increase per square foot | $ / ft²						1
101 Main Street Initial Leases
Commitments And Contingencies [Line Items]
Number of lease extension options | RenewalOption								1
Operating lease term								4 years
Operating lease renewal options period								5 years
Operating lease annual rent for the first year								$ 1,700,000
101 Main Street Additional Leases
Commitments And Contingencies [Line Items]
Number of lease extension options | RenewalOption						1
Operating lease term						5 years 6 months
Operating lease renewal options period						5 years
Operating lease annual rent for the first year						$ 3,500,000