Collaboration Agreements - Additional Information (Detail)					1 Months Ended							3 Months Ended			12 Months Ended
	Mar. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)	Jan. 14, 2014 USD ($)	Oct. 31, 2012 USD ($)	Jan. 31, 2018 USD ($) MilestonePayment	May 31, 2017 USD ($) 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	Mar. 31, 2018 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 43,531,000	$ 16,570,000										$ 43,531,000
Revenue recorded under prior revenue guidance												5,883,000
2018 Restructured Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue					$ 2,900,000
Maximum funding for development and commercialization costs for fitusiran, during transition period					50,000,000
2018 Restructured Agreement | Prior Revenue Standard
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue					25,800,000
2018 Restructured Agreement | Fitusiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount of costs expected to be incurred												43,600,000
Recognized costs under proportional performance model												16,000,000
2018 Restructured Agreement | Fitusiran | Prior Revenue Standard
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred revenue					23,400,000
Incremental to transaction price												22,800,000
Revenue recorded under prior revenue guidance												24,600,000
Sanofi Genzyme | Product Alliances
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone														$ 11,000,000
Upfront fee received				$ 22,500,000
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 | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					15.00%
Sanofi Genzyme | TTR License | Patisiran | Japan
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					25.00%
Sanofi Genzyme | TTR License | Patisiran | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					25.00%
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 | AT3 License Terms
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 | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					30.00%
Sanofi Genzyme | AT3 License Terms | Minimum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate					15.00%
Sanofi Genzyme
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Amount earned upon achievement of milestone												$ 50,000,000		11,000,000
Upfront fee received				$ 22,500,000
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 | 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]
Transaction price		147,300,000
Sanofi Genzyme | Product Alliances
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 | TTR License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Transaction price		127,600,000
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 | Patisiran
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cost-share reimbursements, net of payments including transaction price		$ 63,600,000
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	$ 45,000,000