Fair Value Measurements - Additional Information (Detail) $ in Millions	Apr. 18, 2018 USD ($)	Sep. 30, 2018 USD ($)
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term debt		$ 30.1
Dicerna Pharmaceuticals, Inc
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Additional payment due from litigation settlement	$ 13.0
Litigation settlement agreement payment due date for additional payment	Apr. 18, 2022
Dicerna Pharmaceuticals, Inc | Measurement Input, Present Value Discount Interest Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate	0.11
Dicerna Pharmaceuticals, Inc | Level 2 | Long-term Other Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long-term other assets		$ 9.0