Commitments and Contingencies - Additional Information (Detail)					1 Months Ended	12 Months Ended
	Sep. 27, 2018	Aug. 14, 2018 USD ($) RenewalOption	Apr. 18, 2018 USD ($) shares	Apr. 29, 2016 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2022 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2018 USD ($)	Apr. 30, 2016 a	Mar. 09, 2015 ft²	Sep. 26, 2003 ft²
Commitments And Contingencies [Line Items]
Property, plant and equipment, net					$ 181,900,000			$ 272,652,000
Restricted investments					30,000,000			44,825,000
Litigation settlement, common stock issuable from other party | shares			983,208
Dicerna Pharmaceuticals, Inc
Commitments And Contingencies [Line Items]
Litigation settlement, settlement agreement date			April 18, 2018
Litigation settlement amount			$ 25,000,000
Upfront cash payment			$ 2,000,000
Litigation settlement, common stock issuable from other party | shares			983,208
Litigation settlement, common stock value			$ 10,000,000
Additional payment due from litigation settlement			$ 13,000,000
Litigation settlement payment period			4 years
Litigation settlement agreement payment due date for additional payment			Apr. 18, 2022
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%
Restricted investments					30,000,000			30,000,000
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%
Norton, Massachusetts
Commitments And Contingencies [Line Items]
Undeveloped land acquired | a									12
Norton, Massachusetts | Land and costs related to the construction of manufacturing facility
Commitments And Contingencies [Line Items]
Property, plant and equipment, net					$ 140,500,000			$ 199,300,000
Maximum | Dicerna Pharmaceuticals, Inc | Oligonucleotide
Commitments And Contingencies [Line Items]
Restriction period in development and other activities defined set of targets			4 years
Minimum | Dicerna Pharmaceuticals, Inc | Oligonucleotide
Commitments And Contingencies [Line Items]
Restriction period in development and other activities defined set of targets			18 months
Third Street Lease
Commitments And Contingencies [Line Items]
Area of office and laboratory space held under lease | ft²											129,000
Lease extended expiration date		Jan. 31, 2034			Sep. 30, 2021
Lease extension period		12 years 4 months
Number of lease extension options | RenewalOption		2
Amended lease extension period		5 years
Third Street Lease | Scenario Forecast
Commitments And Contingencies [Line Items]
Facility lease annual rent						$ 10,500,000
Facility lease percentage of annual rent increase thereafter						2.50%
Third Street Lease | Maximum
Commitments And Contingencies [Line Items]
Tenant improvement allowance from landlord		$ 8,400,000
101 Main Street Leases
Commitments And Contingencies [Line Items]
Area of office and laboratory space held under lease | ft²										23,350
Lease extended expiration date	Mar. 31, 2024
Lease extension period	5 years
Amended lease extension period	5 years
Lease expiration date					Mar. 31, 2019
101 Main Street Leases | Scenario Forecast
Commitments And Contingencies [Line Items]
Facility lease annual rent							$ 2,100,000
Facility lease percentage of annual rent increase thereafter							2.00%