Selected Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								10 Months Ended	12 Months Ended																					263 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, 1994	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, 2003	Dec. 31, 2002	Dec. 31, 2001	Dec. 31, 2000	Dec. 31, 1999	Dec. 31, 1998	Dec. 31, 1997	Dec. 31, 1996	Dec. 31, 1995	Feb. 28, 1994	Feb. 28, 1993	Feb. 29, 1992	Dec. 31, 2012
Interim Reporting [Line Items]
Net loss	$ (51,781)	$ (42,834)	$ (36,578)	$ (38,173)	$ (36,397)	$ (38,402)	$ (44,480)	$ (41,525)	$ (2,004)	$ (169,366)	$ (160,804)	$ (170,560)	$ (220,104)	$ (303,039)	$ (293,190)	$ (230,548)	$ (114,338)	$ (75,992)	$ (65,879)	$ (206,265)	$ (48,245)	$ (24,661)	$ (5,679)	$ (3,331)	$ (2,280)	$ (2,570)	$ (2,815)	$ (1,156)	$ (1,175)	$ (911)	$ (2,104,912)
Net loss applicable to common stockholders	$ (51,781)	$ (42,834)	$ (36,578)	$ (38,173)	$ (36,397)	$ (38,402)	$ (44,480)	$ (41,525)		$ (169,366)	$ (160,804)	$ (170,560)																			$ (2,128,124)
Net loss per share applicable to common stockholders - basic and diluted	$ (0.23)	$ (0.22)	$ (0.23)	$ (0.27)	$ (0.30)	$ (0.31)	$ (0.37)	$ (0.34)		$ (0.94)	$ (1.32)	$ (1.50)
Weighted average common shares used to compute basic and diluted net loss per share applicable to common stockholders	229,234	190,534	159,859	143,154	122,357	122,130	121,708	121,057		180,855	121,817	113,672