Collaboration Agreements - Additional Information (Detail)				1 Months Ended								3 Months Ended					9 Months Ended		12 Months Ended
	Jan. 01, 2018 USD ($)	Jan. 14, 2014 USD ($)	Oct. 31, 2012 USD ($)	Jan. 31, 2018 USD ($) MilestonePayment	Nov. 30, 2017 $ / shares shares	May 31, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($)	Feb. 29, 2016 USD ($) shares	Jan. 31, 2015 USD ($) shares	Mar. 31, 2014 USD ($) shares	Feb. 28, 2014 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 16,570,000											$ 5,067,000					$ 5,067,000
Revenues recognized												2,069,000		$ 17,096,000			53,875,000	$ 51,988,000
Issuance of common stock, net of offering costs (in shares) | shares					6,440,000	5,000,000
Proceeds from issuance of common stock to Sanofi Genzyme																		$ 21,381,000
Common stock price | $ / shares					$ 125.00	$ 71.87
2018 Restructured Agreement Related to TTR Programs
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	2,900,000
2018 Restructured Agreement Related to TTR Programs | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	25,800,000
2018 Restructured Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum funding for development and commercialization costs for fitusiran, during transition period												50,000,000					$ 50,000,000
2018 Restructured Agreement | Fitusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total Cost Incurred in Transition												$ 38,000,000
2018 Restructured Agreement | Fitusiran | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 23,400,000
Incremental to transaction price	22,800,000
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received			$ 22,500,000
Amount earned upon achievement of milestone																$ 11,000,000
Unearned milestones included in transaction price	0
Issuance of common stock, net of offering costs (in shares) | shares										344,448	8,766,338
Proceeds from issuance of common stock to Sanofi Genzyme										$ 23,000,000	$ 700,000,000
Common stock price | $ / shares											$ 85.72
Fair value of shares issued											$ 751,500,000
Excess of fair value over cash received for stock issuance											51,500,000
Provision for (Benefit from) income taxes											$ (15,200,000)
Percentage ownership interest owned by noncontrolling owners												11.00%					11.00%
Sanofi Genzyme | Concurrent Private Placement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares						297,501			744,566
Proceeds from issuance of common stock to Sanofi Genzyme						$ 21,400,000			$ 70,700,000
Sanofi Genzyme | Compensatory Purposes
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares								205,030	196,251
Proceeds from issuance of common stock to Sanofi Genzyme								$ 14,300,000	$ 18,300,000
Sanofi Genzyme | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	3,500,000
Sanofi Genzyme | Revusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone																			$ 25,000,000
Sanofi Genzyme | Fitusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost-share reimbursements, net of payments including transaction price	147,300,000
Sanofi Genzyme | Product Alliances
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received			$ 22,500,000
Amount earned upon achievement of milestone																11,000,000
Deferred revenue																$ 33,500,000
Sanofi Genzyme | Regional Collaboration Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in development cost		20.00%
Maximum number of potential future milestones		$ 75,000,000
Potential future payment for the achievement of certain development milestones		55,000,000
Potential future payment for the achievement of specified commercialization milestones		$ 20,000,000
Royalty rate		20.00%
Sanofi Genzyme | Co-developed/ Co-commercialized Collaboration Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in development cost		50.00%
Royalty rate		20.00%
Received incremental share of co-development costs for exercise of right, fitusiran							$ 6,000,000
Expected first milestone payment to be received fitusiran																				$ 25,000,000
Sanofi Genzyme | Co-developed/ Co-commercialized Collaboration Product | Revusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone															$ 25,000,000
Sanofi Genzyme | Global Collaboration Product
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sharing in development cost		100.00%
Maximum number of potential future milestones		$ 200,000,000
Potential future payment for the achievement of certain development milestones																	$ 100,000,000
Potential future payment for the achievement of specified commercialization milestones																	100,000,000
Royalty rate		20.00%
Sanofi Genzyme | AT3 License Terms and the Exclusive TTR License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum funding for development and commercialization costs for fitusiran, during transition period				$ 50,000,000
Sanofi Genzyme | TTR License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price	127,600,000
Sanofi Genzyme | TTR License | ONPATTRO | Japan
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				25.00%
Sanofi Genzyme | TTR License | ONPATTRO | Maximum | Excluding United States, Canada and Western Europe
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				25.00%
Sanofi Genzyme | TTR License | Revusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost-share reimbursements, net of payments including transaction price	57,000,000
Sanofi Genzyme | TTR License | TTRsc02 | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				30.00%
Sanofi Genzyme | TTR License | TTRsc02 | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				15.00%
Sanofi Genzyme | TTR License | Back-up Products | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				15.00%
Sanofi Genzyme | TTR License | Patisiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost-share reimbursements, net of payments including transaction price	63,600,000
Sanofi Genzyme | AT3 License Terms
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of milestone payments required | MilestonePayment				1
Expected milestone payment to be received fitusiran				$ 50,000,000
Sanofi Genzyme | AT3 License Terms | Back-up Products | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				15.00%
Sanofi Genzyme | AT3 License Terms | Fitusiran | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				30.00%
Sanofi Genzyme | AT3 License Terms | Fitusiran | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate				15.00%
Sanofi Genzyme | 2018 Restructured Agreement | Fitusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone													$ 50,000,000
Deferred revenue	$ 600,000
Transaction price																	37,600,000
Revenues recognized												$ (1,600,000)					37,600,000
Sanofi Genzyme | 2018 Restructured Agreement | Fitusiran | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues recognized												$ (900,000)					$ 60,300,000