Stock Based Awards - Schedule of Estimated Stock-Based Compensation Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended			73 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 4,641	$ 215	$ 141	$ 5,189
Research and development [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,230	124	91	1,599
General and administrative [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 3,411	$ 91	$ 50	$ 3,590