Stockholders' Equity (Summary of Stock Option Activity and Weighted Average Exercise Prices) (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Options
Beginning balance (in shares) | shares	7,959
Granted (in shares) | shares	1,378
Exercised (in shares) | shares	(735)
Cancelled or expired (in shares) | shares	(174)
Ending balance (in shares) | shares	8,428
Options vested and expected to vest (in shares) | shares	7,355
Options exercisable at end of period (in shares) | shares	4,661
Weighted Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 10.71
Granted (in dollars per share) | $ / shares	12.82
Exercised (in dollars per share) | $ / shares	8.95
Cancelled or expired (in dollars per share) | $ / shares	9.22
Ending balance (in dollars per share) | $ / shares	11.24
Options vested and expected to vest (in dollars per share) | $ / shares	11.22
Options exercisable at end of period (in dollars per share) | $ / shares	$ 11.43
Weighted Average Remaining Contractual Term (in years)
Options outstanding at end of period	6 years 5 months 17 days
Options vested and expected to vest	6 years 15 days
Options exercisable at end of period	4 years 3 months 28 days
Aggregate Intrinsic Value
Options outstanding at end of period | $	$ 43,795
Options vested and expected to vest | $	38,459
Options exercisable at end of period | $	$ 23,614