Stockholders' Equity (Summary of Stock Option Activity and Weighted Average Exercise Prices) (Details) (USD $)	6 Months Ended
	Jun. 30, 2014
Options
Options outstanding at December 31, 2013	9,724,193
Options granted	2,833,500
Options exercised	(286,553)
Options cancelled	(836,916)
Options outstanding at June 30, 2014	11,434,224
Options exercisable at June 30, 2014	4,512,864
Weighted Average Exercise Price
Options outstanding at December 31, 2013	$ 10.86
Options granted	$ 10.72
Options exercised	$ 8.40
Options cancelled	$ 13.27
Options outstanding at June 30, 2014	$ 10.71
Options exercisable at June 30, 2014	$ 9.15