Significant Agreements - Additional Information (Detail)					1 Months Ended										3 Months Ended										9 Months Ended	12 Months Ended				47 Months Ended
	Jan. 01, 2018 USD ($)	Jan. 06, 2015 USD ($) Program License	Jan. 14, 2014 USD ($)	Oct. 31, 2012 USD ($)	Jan. 31, 2018 USD ($) MilestonePayment	Nov. 30, 2017 $ / shares shares	May 31, 2017 $ / shares shares	Jan. 31, 2017 USD ($)	Feb. 29, 2016 USD ($) shares	Jul. 31, 2015	Jan. 31, 2015 USD ($) shares	Mar. 31, 2014 USD ($) shares	Feb. 28, 2014 USD ($) $ / shares shares	Feb. 28, 2013 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($)	Jan. 06, 2018 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 16,570,000														$ 3,954,000											$ 3,954,000
Revenues recognized															21,033,000	$ 2,069,000	$ 29,907,000	$ 21,899,000	$ 37,924,000	$ 17,096,000	$ 15,932,000	$ 18,960,000				74,908,000	$ 89,912,000	$ 47,159,000
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	14,301,000
Underwritten public offering amount per share | $ / shares						$ 125.00	$ 71.87
Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fee																										8,000,000	7,700,000	$ 2,400,000
Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement termination notice period										90 days
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
Product Alliances | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received	25,000,000													$ 25,000,000
Amount earned upon achievement of milestone	30,000,000																		20,000,000				$ 10,000,000			30,000,000
Maximum number of potential future milestones														180,000,000
Potential future payment for the achievement of certain development milestones														30,000,000
Potential future payment for the achievement of specified commercialization milestones														100,000,000
Transaction price	72,900,000
Cost-share reimbursements, net of payments included in transaction price	17,900,000
Potential future payment for the achievement of specified regulatory milestones														$ 50,000,000
Next potential milestone payment																										25,000,000
Milestones earned included in transaction price	0
Cost incurred																														$ 17,800,000
Total costs expected															$ 17,900,000											$ 17,900,000
Product Alliances | Maximum | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated range of expiration for fundamental patents																										2028
Product Alliances | Minimum | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated range of expiration for fundamental patents																										2016
Product Alliances | Prior Revenue Standard | ASU 2014-09 Revenue from Contracts with Customers | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	5,700,000
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
2018 Restructured Agreement | Fitusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total Cost Incurred in Transition																									$ 38,000,000
2018 Restructured Agreement | Prior Revenue Standard | Fitusiran | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Incremental to transaction price	22,800,000
Discovery and Development Alliances | Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of programs Providing exclusive RNA therapeutic license rights | Program		4
Potential future payment upon the occurrence of achievement of specified development and regulatory milestones		$ 3,400,000
Number of licensed products | License		10
Additional number of licensed products | License		1
Discovery and Development Alliances | Ionis | Up Front Payment | Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fee		$ 5,000,000
Discovery and Development Alliances | Ionis | Milestone Payments | Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future payment upon the occurrence of achievement of specified development and regulatory milestones		3,400,000
Discovery and Development Alliances | Ionis | Therapeutic target | Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fee		$ 500,000
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
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														21,400,000
Voting percentage													20.00%
Minimum percentage of ownership interest terminate													7.50%
Agreement lock-up period, extension term													2 years
Agreement lock-up period													6 months
Percentage of acquired outstanding common stock for Genzyme to appoint individual to board of directors													20.00%
Underwritten public offering amount per share | $ / 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
Sanofi Genzyme | Alnylam Pharmaceuticals Inc
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage ownership interest owned by noncontrolling owners															10.00%											10.00%
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											$ 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 | Following the two-year anniversary
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of trailing days on which average is calculated													10 days
Sanofi Genzyme | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Agreement to acquire outstanding shares of common stock percentage													30.00%
Sanofi Genzyme | Maximum | Following the two-year anniversary
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of sale of initial shares													25.00%
Sanofi Genzyme | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Agreement to acquire outstanding shares of common stock percentage													5.00%
Percentage of ownership interest													30.00%
Sanofi Genzyme | Minimum | Following the two-year anniversary
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of market price per share of common stock compared to share of common stock at closing of stock purchase													100.00%
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 included in 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
Sanofi Genzyme | Product Alliances | Prior Revenue Standard
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
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								$ 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 U.S., 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 included in 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 included in 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																										37,600,000
Sanofi Genzyme | 2018 Restructured Agreement | Prior Revenue Standard | Fitusiran | ASU 2014-09 Revenue from Contracts with Customers
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenues recognized																										$ 60,300,000