Effect on Net Loss and Earnings per Share if Applied Fair Value Recognition Provisions to Stock-Based Awards (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								4 Months Ended	12 Months Ended									28 Months Ended	112 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, 2003	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2006	Dec. 31, 2005	Dec. 31, 2004	Dec. 31, 2005	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net loss	$ (30,225)	$ (17,824)	$ (23,613)	$ (24,470)	$ (13,244)	$ (802)	$ (10,724)	$ (39,008)	$ (160)	$ (96,132)	$ (63,778)	$ (32,670)	$ (7,649)	$ (25,231)	$ (26,608)	$ (17,857)	$ (9,517)	$ (5,687)	$ (15,364)	$ (283,724)
Stock-based compensation expense										(4,880)	(2,759)	(3,637)								(20,181)
Basic and diluted net loss per share:
Basic and diluted net loss per share	$ (0.37)	$ (0.23)	$ (0.30)	$ (0.32)	$ (0.19)	$ (0.01)	$ (0.16)	$ (0.65)		$ (1.22)	$ (0.97)	$ (0.71)							$ (3.75)
Included in reported net loss
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense																			802
Under the fair-value based method
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense																			(1,756)
Pro Forma
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net loss																			$ (16,318)
Basic and diluted net loss per share:
Basic and diluted net loss per share																			$ (3.98)