Computation of Net Loss Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Computation Of Earnings Per Share Line Items
Net loss	$ (7,067)	$ (3,563)	$ (2,988)	$ (6,841)	$ (10,019)	$ (6,158)	$ (1,752)	$ (3,172)	$ (20,459)	$ (21,101)	$ (13,423)
Basic and diluted shares:
Weighted average common shares outstanding									64,932	57,834	35,439
Weighted average unvested common shares subject to restrictions									(614)	(316)	0
Weighted average shares used to compute basic and diluted net loss per share									64,318	57,518	35,439
Net loss per share - basic and diluted	$ (0.11)	$ (0.06)	$ (0.05)	$ (0.11)	$ (0.16)	$ (0.10)	$ (0.03)	$ (0.06)	$ (0.32)	$ (0.37)	$ (0.38)