Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)			1 Months Ended				3 Months Ended			9 Months Ended
	Apr. 18, 2018	Jan. 01, 2018	Nov. 30, 2017	May 31, 2017	Mar. 31, 2014	Feb. 28, 2014	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Issuance of common stock, net of offering costs (in shares)			6,440,000	5,000,000
Underwritten public offering amount per share			$ 125.00	$ 71.87
Issuance of common stock, net of offering costs			$ 784,500,000	$ 355,200,000
Underwriting discounts and commissions and other offering expenses			$ 20,500,000	$ 4,200,000
Stockholders' equity, decrease										$ 288,300,000
Gains or losses from sale of available-for-sale debt securities										0	$ 0
Reclassification from AOCI for Available for sale debt securities										$ 0	0
Marketable securities classified as cash equivalents, maximum original maturity										90 days
Policy for marketable securities										90 days
Unrealized gain on equity securities										$ 5,004,000
Litigation settlement, common stock issuable from other party	983,208
Realized loss on sale of marketable equity securities											1,894,000
Accumulated other comprehensive loss, net of tax							$ 33,392,000			33,392,000		$ 34,433,000
Impairment of capitalized inventory										0
Capitalized cost							0			$ 0
Product returns, description										ONPATTRO may be returned if it is damaged, defective or expired, with “expired” defined as having three months or less to expiry or within three months past expiry.
Revenues							2,069,000		$ 17,096,000	$ 53,875,000	51,988,000
Deferred revenue, current portion							3,444,000			3,444,000		41,705,000
Deferred revenue, net of current portion							1,623,000			1,623,000		43,075,000
Gain on litigation settlement										20,564,000
Recognition of deferred revenue										14,953,000
ASU 2014-09 Revenue from Contracts with Customers
Significant Accounting Policies [Line Items]
Deferred revenue, current portion		$ 7,242,000
Deferred revenue, net of current portion		9,328,000
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)
Product Revenues, Net
Significant Accounting Policies [Line Items]
Revenues							460,000			$ 460,000
Minimum
Significant Accounting Policies [Line Items]
Accounts receivable payment term										30 days
Maximum
Significant Accounting Policies [Line Items]
Accounts receivable payment term										40 days
Regulus Therapeutics Inc
Significant Accounting Policies [Line Items]
Realized loss on sale of marketable equity securities											$ 1,900,000
Accumulated other comprehensive loss, net of tax							32,800,000			$ 32,800,000		$ 32,800,000
Dicerna Pharmaceuticals, Inc
Significant Accounting Policies [Line Items]
Unrealized gain on equity securities							3,000,000			5,000,000
Litigation settlement, common stock issuable from other party	983,208
Gain on litigation settlement								$ 20,600,000
Valuation at settlement date of common stock received from litigation settlement	$ 10,000,000
Upfront cash payment	2,000,000
Litigation settlement discounted present value of additional payment	8,600,000
Additional payment due from litigation settlement	$ 13,000,000
Litigation settlement agreement payment due date for additional payment	Apr. 18, 2022
Vir Biotechnology
Significant Accounting Policies [Line Items]
Deferred revenue, current portion							3,400,000			3,400,000
Deferred revenue, net of current portion							$ 1,600,000			$ 1,600,000
Sanofi Genzyme
Significant Accounting Policies [Line Items]
Issuance of common stock, net of offering costs (in shares)					344,448	8,766,338
Underwritten public offering amount per share						$ 85.72
Sanofi Genzyme | ASU 2014-09 Revenue from Contracts with Customers
Significant Accounting Policies [Line Items]
Recognition of deferred revenue		45,700,000
Remaining deferred revenues		3,500,000
Mdco | ASU 2014-09 Revenue from Contracts with Customers
Significant Accounting Policies [Line Items]
Recognition of deferred revenue		4,500,000
Remaining deferred revenues		1,200,000
Kyowa Hakko Kirin | ASU 2014-09 Revenue from Contracts with Customers
Significant Accounting Policies [Line Items]
Recognition of deferred revenue		$ 15,500,000