Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interim Reporting [Line Items]
Net loss	$ (277,020)	$ (31,857)	$ (28,910)	$ (30,658)	$ (36,439)	$ (36,520)	$ (73,365)	$ (52,056)	$ (368,445)	$ (198,382)	$ (191,490)
Net loss per share - basic and diluted	$ (0.66)	$ (0.08)	$ (0.07)	$ (0.08)	$ (0.09)	$ (0.09)	$ (0.19)	$ (0.14)	$ (0.91)	$ (0.51)	$ (0.64)
Weighted average common shares used to compute basic and diluted net loss per share	419,314	405,199	401,018	398,916	396,793	394,163	380,770	368,784