Significant Agreements - Additional Information (Detail)					1 Months Ended														3 Months Ended			9 Months Ended	12 Months Ended				26 Months Ended	36 Months Ended
	Feb. 01, 2016 USD ($) shares	Jan. 22, 2015 USD ($) shares	Jan. 14, 2014 USD ($)	Oct. 31, 2012 USD ($)	Jan. 31, 2018 USD ($) MilestonePayment	Nov. 30, 2017 USD ($) $ / shares shares	May 31, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($)	Oct. 31, 2016	Sep. 28, 2015	Jan. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($) shares	Feb. 28, 2014 USD ($) Right $ / shares shares	Feb. 28, 2013 USD ($)	Aug. 31, 2012 USD ($)	May 31, 2008 USD ($)	Mar. 31, 2004 USD ($) License	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2016	Dec. 31, 2017 USD ($) Program	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock, net of offering costs (in shares) | shares						6,440,000	5,000,000				5,447,368
Proceeds from issuance of common stock to Sanofi Genzyme																							$ 21,381,000	$ 14,301,000	$ 89,018,000
Common stock price | $ / shares						$ 125.00	$ 71.87				$ 95.00
Provision for (Benefit from) income taxes																							0	0	0
Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fee																							$ 7,700,000	2,400,000	3,500,000
Ionis
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative agreement termination notice period																							90 days
Maximum | Monsanto Alliance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated range of expiration for fundamental patents																							2025
Minimum | Monsanto Alliance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated range of expiration for fundamental patents																							2016
Product Alliances | MDCO
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received															$ 25,000,000
Amount earned upon achievement of milestone						$ 20,000,000													$ 20,000,000	$ 10,000,000
Deferred revenue																			5,700,000				$ 5,700,000					$ 5,700,000
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
Potential future payment for the achievement of specified regulatory milestones															$ 50,000,000
Next potential milestone payment																							25,000,000
Revenue from reimbursement of costs incurred																												12,200,000
Revenue recognizing period															5 years
Product Alliances | MDCO | Licensing Agreements
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from reimbursement of costs incurred																							$ 5,400,000	3,000,000	3,800,000
Product Alliances | MDCO | Phase 1 Clinical Trial
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone																										$ 10,000,000
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
Platform Alliances | Monsanto Alliance
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received																$ 29,200,000
Amount earned upon achievement of milestone																											$ 4,000,000
Deferred revenue																			2,500,000				$ 2,500,000					2,500,000
Maximum number of potential future milestones																$ 5,000,000
Next potential milestone payment																							0
Revenue recognizing period																5 years
Period of exclusivity in the collaboration																10 years
Final required milestone payment cancelled												$ 1,000,000
Deferred revenue, potential refundable amount																$ 5,000,000			2,500,000				2,500,000					2,500,000
Period for services under contract																5 years
Contractual agreement obligation period																30 months
Additional revenue, recognized																							2,500,000
Deferred revenue potential refundable payment additional																			2,500,000				2,500,000					2,500,000
Platform Alliances | Monsanto Alliance | Amended Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue												$ 16,800,000															$ 16,800,000
Platform Alliances | Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee received																	$ 100,000,000
Deferred revenue																			0				0					0
Maximum number of potential future milestones																	171,000,000
Potential future payment for the achievement of certain development milestones																	26,000,000
Potential future payment for the achievement of specified commercialization milestones																	105,000,000
Potential future payment for the achievement of specified regulatory milestones																	$ 40,000,000
Next potential milestone payment																							$ 2,000,000
Granted period of royalty bearing license																	5 years
Milestone fees to company upon achievement of specified technology transfer milestones																	$ 50,000,000
License and collaborations agreements, prior written notice period before termination																	180 days
Platform Alliances | Takeda | Joint Technology Transfer Committee
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum life of collaboration committee																	7 years
Platform Alliances | Takeda | Joint Delivery Collaboration Committee
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Maximum life of collaboration committee																	7 years
Platform Alliances | Maximum | Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated range of expiration for fundamental patents																							2025
Platform Alliances | Minimum | Takeda
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated range of expiration for fundamental patents																							2016
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 | Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fee																							$ 200,000	200,000	1,900,000
License fees incurred but not yet paid																							1,000,000	200,000
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
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License fee																		$ 500,000
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone																										$ 25,000,000
Deferred revenue																			$ 49,168,000				$ 49,168,000					$ 49,168,000
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
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%
Common stock price | $ / shares														$ 85.72
Fair value of shares issued														$ 751,500,000
Excess of fair value over cash received for stock issuance														51,450,000
Provision for (Benefit from) income taxes														$ (15,200,000)
Percentage ownership interest owned by noncontrolling owners																			11.00%				11.00%					11.00%
Revenue from reimbursement of costs incurred																							$ 51,846,000	$ 54,337,000	$ 33,949,000
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 | 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 | 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 | Product Alliances | Patisiran and Revusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Performance period									5 years													6 years
Sanofi Genzyme | Product Alliances | Fitusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Performance period										5 years													6 years
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					$ 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 | Subsequent Event
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 | Subsequent Event | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					15.00%
Sanofi Genzyme | TTR License | Patisiran | Subsequent Event | Japan
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					25.00%
Sanofi Genzyme | TTR License | Patisiran | Subsequent Event | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					25.00%
Sanofi Genzyme | TTR License | TTRsc02 | Subsequent Event | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					30.00%
Sanofi Genzyme | TTR License | TTRsc02 | Subsequent Event | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					15.00%
Sanofi Genzyme | AT3 License Terms | Subsequent Event
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					15.00%
Number of milestone payments required | MilestonePayment					1
Expected milestone payment to be received fitusiran					$ 50,000,000
Sanofi Genzyme | AT3 License Terms | Subsequent Event | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					30.00%
Sanofi Genzyme | AT3 License Terms | Subsequent Event | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					15.00%
Sanofi Genzyme | Investors
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of demand rights to conduct registered underwritten public offering | Right														3