Convertible Preferred Stock (Details) (Initial public offering, USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Common stock
Convertible preferred stock
Aggregate gross proceeds from an initial public offering that would trigger automatic conversion of preferred shares to common shares	$ 50
Common stock | Minimum
Convertible preferred stock
Minimum offering price per share in an initial public offering to trigger automatic conversion of preferred shares to common shares	$ 4.40
Minimum offering price per share in an initial public offering expressed as a percentage of the Series C conversion price to trigger automatic conversion of preferred shares to common shares	250.00%
Series B Preferred Stock
Convertible preferred stock
Dividend to be declared and paid in cash on automatic conversion of preferred shares	$ 4.3