Basis of Presentation - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Basis Of Presentation [Line Items]
Cash flows from operating activities	$ (44,045,000)	$ (22,126,000)
Clinical trial supplies written down	8,200,000
Clinical trial supplies included in other current assets			4,600,000
In-process API held at the manufacturer			1,600,000
Drug product located at depots			3,000,000
Cumulative amortization costs	1,000,000
Prepaid expenses related to clinical agreements			1,100,000
Upfront payments	3,900,000		4,500,000
Prepaid costs applied to final invoices	2,800,000		2,800,000
License obligations	8,259,000		5,524,000
Archemix Corporation [Member]
Basis Of Presentation [Line Items]
License obligations	1,000,000
Upfront Contract [Member]
Basis Of Presentation [Line Items]
Other non-current assets include upfront contract signing costs			1,700,000
Upfront contract signing costs amortized period			3 years
Upfront Contract [Member] | Scenario Forecast [Member]
Basis Of Presentation [Line Items]
Other current assets include upfront contract signing costs	$ 1,100,000