Summary of Significant Accounting Policies - Basic and Diluted Net Loss Per Share (Detail) (USD $) In Thousands, except Per 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
Numerator
Net income (loss)	$ (20,521)	$ (12,825)	$ 62,865	$ (20,932)	$ (35,617)	$ (10,852)	$ (13,332)	$ (21,055)	$ 8,587	$ (80,856)	$ (47,386)
Denominator
Weighted average common shares outstanding, basic	150,016	141,045	139,985	139,309					142,607	108,568	80,558
Common stock warrants									2,439	0	0
Weighted average common shares outstanding, diluted	150,040	141,045	139,985	145,323					145,046	108,568	80,558
Basic and diluted net loss per share (in dollars per share)					$ (0.28)	$ (0.10)	$ (0.13)	$ (0.21)	$ 0.06	$ (0.74)	$ (0.59)