Stock-Based Compensation (Summary of Option Activity) (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Number of Options
Beginning balance (in shares) | shares	11,426,296
Granted (in shares) | shares	368,894
Exercised (in shares) | shares	(1,003,735)
Forfeited (in shares) | shares	(1,075,772)
Expired (in shares) | shares	(555,889)
Ending balance (in shares) | shares	9,159,794
Exercisable, number of options (in shares) | shares	6,346,632
Vested or expected to vest, number of options (in shares) | shares	9,066,131
Weighted- Average Exercise Price
Beginning balance, weighted-average exercise price (in dollars per share) | $ / shares	$ 6.10
Granted (in dollars per share) | $ / shares	4.13
Exercised (in dollars per share) | $ / shares	0.93
Forfeited (in dollars per share) | $ / shares	9.00
Expired (in dollars per share) | $ / shares	11.74
Ending balance, weighted-average exercise price (in dollars per share) | $ / shares	5.90
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares	5.09
Vested or expected to vest, weighted-average exercise price (in dollars per share) | $ / shares	$ 5.89
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual term	6 years 11 months 23 days
Outstanding, aggregate intrinsic value | $	$ 15,276
Exercisable, weighted-average remaining contractual term	6 years 3 months 18 days
Exercisable, aggregate intrinsic value | $	$ 14,670
Vested or expected to vest, weighted-average remaining contractual term	6 years 11 months 23 days
Vested or expected to vest, aggregate intrinsic value | $	$ 15,271