Research and Development Arrangements - Additional Information (Details) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 27, 2018	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014	Jun. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accumulated deficit incurred			$ (714,403,000)		$ (714,403,000)		$ (646,081,000)
Deferred revenue, current			0		0		9,572,000
Research and development			21,391,000	$ 24,947,000	65,858,000	$ 64,045,000
Contract assets			5,876,000		5,876,000		0
Accounts receivable			9,156,000		9,156,000		1,112,000
2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Non-refundable option fee								$ 15,000,000
2016 Astellas Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with Customer, Liability, Revenue Recognized			(1,063,000)		672,000
Contract assets			5,876,000		5,876,000		19,413,000			$ 10,375,000
Maximum [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research milestone payments					2,000,000
Maximum [Member] | 2016 Astellas Amendment [Member] | Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					200,000,000
Accounting Standards Update 2014-09 [Member] | Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Pre-commercialization milestone payments eligible to receive			300,000,000		300,000,000
Accounting Standards Update 2014-09 [Member] | Maximum [Member] | Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Commercialization milestone payments eligible to receive			300,000,000		300,000,000
Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accumulated deficit incurred		$ (11,500,000)
Contract liabilities			4,400,000		4,400,000
Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Accumulated deficit incurred		19,400,000
Non-refundable fees		50,000,000
Research and development		35,600,000
Contract assets		19,400,000
License transaction price		83,000,000
Services transaction price		18,000,000
Amgen [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option exercised amount	$ 40,000,000
Percentage of incremental royalty receivable on annual net sales	4.00%
Co-invest option payment	$ 6,300,000				18,800,000
Reduction in research and development revenue due to payments made to fund the Co-Invest Option	$ 0						20,000,000	1,300,000
Amgen [Member] | Accounting Standards Update 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Co-invest option payment			6,300,000		18,800,000
Revenue from Contract with Customer, Excluding Assessed Tax						1,300,000
Co-invest option payment						13,800,000
Amgen [Member] | Accounting Standards Update 2014-09 [Member] | Japan [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0			0	10,000,000
Amgen [Member] | Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contract with Customer, Liability, Revenue Recognized		18,800,000
Accumulated deficit incurred		$ (18,800,000)
Astellas [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			8,526,000	$ 2,112,000	16,791,000	$ 8,810,000
Astellas [Member] | 2014 Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received									$ 30,000,000
Amount received as milestone payment									$ 15,000,000
Astellas [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount received as milestone payment								15,000,000
Revenue recognition, Non-refundable upfront amendment fee								35,000,000
Revenue Recognition over Performance Period								44,200,000
Allocated Consideration								$ 94,200,000
Astellas [Member] | 2016 Astellas Agreement [Member] | Tirasemtiv License | Tirasemtiv Option
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized for milestones achieved							15,000,000
Astellas [Member] | Maximum [Member] | 2016 Astellas Amendment [Member] | Non Neuromuscular Indications
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					95,000,000
Astellas [Member] | Minimum [Member] | 2016 Astellas Amendment [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					600,000,000
Astellas [Member] | Minimum [Member] | 2016 Astellas Amendment [Member] | SMA and Other Neuromuscular Indications [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential amount receivable under collaboration agreement					100,000,000
Astellas [Member] | Calculated Under Revenue Guidance in Effect Before Topic 606 [Member] | 2016 Astellas Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, current							15,000,000
Astellas [Member] | Accounting Standards Update 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Unrecognized transaction price			9,500,000		9,500,000		0
Accounts receivable			$ 9,200		$ 9,200		$ 0