Warrants - Changes in Fair Value of Warrant Liability (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Class of Warrant or Right [Line Items]
Warrant liability, Beginning balance	$ 10,964
Fair value of warrants exercised	(124)
Change in fair value of warrants	5,003	18,270
Warrant liability, Ending balance	15,843
December 2012 Warrants [Member]
Class of Warrant or Right [Line Items]
Warrant liability, Beginning balance	6,954
Fair value of warrants exercised
Change in fair value of warrants	3,293
Warrant liability, Ending balance	10,247
April 2011 Warrants [Member]
Class of Warrant or Right [Line Items]
Warrant liability, Beginning balance	3,310
Fair value of warrants exercised	(124)
Change in fair value of warrants	1,306
Warrant liability, Ending balance	4,492
March 2011 Warrants [Member]
Class of Warrant or Right [Line Items]
Warrant liability, Beginning balance	378
Fair value of warrants exercised
Change in fair value of warrants	185
Warrant liability, Ending balance	563
March 2010 Warrants [Member]
Class of Warrant or Right [Line Items]
Warrant liability, Beginning balance	183
Fair value of warrants exercised
Change in fair value of warrants	147
Warrant liability, Ending balance	330
August 2009 Warrants [Member]
Class of Warrant or Right [Line Items]
Warrant liability, Beginning balance	139
Fair value of warrants exercised
Change in fair value of warrants	72
Warrant liability, Ending balance	$ 211