Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net (loss) income	$ (39,256)	$ (35,793)	$ (31,350)	$ (30,726)	$ (25,062)	$ (18,550)	$ (21,598)	$ (18,142)	$ (137,125)	$ (83,352)	$ 11,366
Noncumulative dividends on convertible preferred stock											(11,366)
Net loss attributable to common stockholders, basic									(137,125)	(83,352)
Net loss attributable to common stockholders, diluted									$ (137,125)	$ (83,352)
Shares used to compute net (loss) income per share attributable to common stockholders:
Weighted average common shares outstanding									42,977,463	22,842,443	1,350,939
Weighted average effect of dilutive stock options											697,928
Weighted average number of shares outstanding, diluted shares									42,977,463	22,842,443	2,048,867
Net loss per share attributable to common stockholders:
Basic									$ (3.19)	$ (3.65)
Diluted									$ (3.19)	$ (3.65)