Stock-Based Compensation (Summary of Option Activity) (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 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	(1,351,648)
Forfeited (in shares) | shares	(1,095,455)
Expired (in shares) | shares	(1,120,351)
Ending balance (in shares) | shares	8,227,736
Exercisable, number of options (in shares) | shares	5,948,010
Vested or expected to vest, number of options (in shares) | shares	8,164,605
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	0.87
Forfeited (in dollars per share) | $ / shares	9.07
Expired (in dollars per share) | $ / shares	11.36
Ending balance, weighted-average exercise price (in dollars per share) | $ / shares	5.76
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares	4.98
Vested or expected to vest, weighted-average exercise price (in dollars per share) | $ / shares	$ 5.75
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, weighted-average remaining contractual term	6 years 8 months 19 days
Outstanding, aggregate intrinsic value | $	$ 13,831
Exercisable, weighted-average remaining contractual term	6 years 29 days
Exercisable, aggregate intrinsic value | $	$ 13,510
Vested or expected to vest, weighted-average remaining contractual term	6 years 8 months 19 days
Vested or expected to vest, aggregate intrinsic value | $	$ 13,827