Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)			3 Months Ended
	Apr. 18, 2018	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Stockholders' equity, decrease			$ 11,600,000
Realized loss on sale of marketable securities				$ 1,549,000
Marketable securities classified as cash equivalents, maximum original maturity			90 days
Policy for marketable securities			90 days
Accumulated other comprehensive loss, net of tax			$ 34,853,000		$ 34,433,000
Deferred revenue		$ 7,242,000	35,855,000		41,705,000
Deferred revenue, net of current portion		9,328,000	7,676,000		43,075,000
Capitalized cost			0
Recognition of deferred revenue			5,883,000
Dicerna | Subsequent Event
Significant Accounting Policies [Line Items]
Litigation settlement, common stock issuable from other party	983,208
ASU 2014-09 Revenue from Contracts with Customers | Sanofi Genzyme
Significant Accounting Policies [Line Items]
Recognition of deferred revenue		45,700,000
Remaining deferred revenues		3,500,000
ASU 2014-09 Revenue from Contracts with Customers | Mdco
Significant Accounting Policies [Line Items]
Recognition of deferred revenue		4,500,000
Remaining deferred revenues		1,200,000
ASU 2014-09 Revenue from Contracts with Customers | Kyowa Hakko Kirin
Significant Accounting Policies [Line Items]
Recognition of deferred revenue		15,500,000
ASU 2014-09 Revenue from Contracts with Customers | Reduction In Deferred Revenue
Significant Accounting Policies [Line Items]
Offsetting adjustments to accumulated deficit			(68,200,000)
ASU 2014-09 Revenue from Contracts with Customers | Reduction In Deferred Tax Asset
Significant Accounting Policies [Line Items]
Offsetting adjustments to accumulated deficit		$ (13,600,000)
Regulus Therapeutics Inc
Significant Accounting Policies [Line Items]
Accumulated other comprehensive loss, net of tax			$ 32,800,000		$ 32,800,000