Summary of Significant Accounting Policies - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 198,014,000	$ 170,566,000	$ 181,582,000	$ 13,288,000
Accumulated deficit	(173,311,000)	(111,259,000)
Working capital	$ 193,300,000
Number of operating segment | Segment	1
Cash and cash equivalents maturity period	Three months or less
Impairments of long-lived assets	$ 0	$ 0
Minimum [Member]
Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	5 years
Percentage for the amount of benefit realized upon ultimate settlement	50.00%