Stockholders' Equity (Summary of Stock Option Activity and Weighted Average Exercise Prices) (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Options
Beginning balance (in shares) | shares	7,959
Granted (in shares) | shares	1,952
Exercised (in shares) | shares	(2,874)
Cancelled or expired (in shares) | shares	(554)
Ending balance (in shares) | shares	6,483
Options vested and expected to vest (in shares) | shares	5,614
Options exercisable at end of period (in shares) | shares	3,250
Weighted Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 10.71
Granted (in dollars per share) | $ / shares	15.03
Exercised (in dollars per share) | $ / shares	10.72
Cancelled or expired (in dollars per share) | $ / shares	9.91
Ending balance (in dollars per share) | $ / shares	12.07
Options vested and expected to vest (in dollars per share) | $ / shares	11.80
Options exercisable at end of period (in dollars per share) | $ / shares	$ 11.00
Weighted Average Remaining Contractual Term (in years)
Options outstanding at end of period	6 years 10 months 28 days
Options vested and expected to vest	6 years 6 months 18 days
Options exercisable at end of period	4 years 9 months 11 days
Aggregate Intrinsic Value
Options outstanding at end of period | $	$ 90,038
Options vested and expected to vest | $	79,474
Options exercisable at end of period | $	$ 48,604