Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Dec. 31, 2010
Liabilities measured at fair value on recurring basis, fair value hierarchy of valuation techniques
Common stock warrants, fair value		[1]	$ 16,622	[1]
Level 1 [Member]
Liabilities measured at fair value on recurring basis, fair value hierarchy of valuation techniques
Common stock warrants, fair value		[1]		[1]
Level 2 [Member]
Liabilities measured at fair value on recurring basis, fair value hierarchy of valuation techniques
Common stock warrants, fair value		[1]		[1]
Level 3 [Member]
Liabilities measured at fair value on recurring basis, fair value hierarchy of valuation techniques
Common stock warrants, fair value		[1]	$ 16,622	[1]	$ 26,193

[1] The fair value of common stock warrants is estimated using a Black-Scholes option-pricing model. See Note 11- Convertible Preferred Stock and Stock Warrants for additional information