Stockholders' Equity (Summary of Stock Option Activity and Weighted Average Exercise Prices) (Details) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Options
Beginning balance (in shares) | shares	7,959
Granted (in shares) | shares	1,765
Exercised (in shares) | shares	(2,273)
Cancelled or expired (in shares) | shares	(274)
Ending balance (in shares) | shares	7,177
Options vested and expected to vest (in shares) | shares	6,189
Options exercisable at end of period (in shares) | shares	3,658
Weighted Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 10.71
Granted (in dollars per share) | $ / shares	14.10
Exercised (in dollars per share) | $ / shares	10.73
Cancelled or expired (in dollars per share) | $ / shares	9.49
Ending balance (in dollars per share) | $ / shares	11.58
Options vested and expected to vest (in dollars per share) | $ / shares	11.46
Options exercisable at end of period (in dollars per share) | $ / shares	$ 11.04
Weighted Average Remaining Contractual Term (in years)
Options outstanding at end of period	7 years 11 days
Options vested and expected to vest	6 years 7 months 28 days
Options exercisable at end of period	4 years 11 months 28 days
Aggregate Intrinsic Value
Options outstanding at end of period | $	$ 68,433
Options vested and expected to vest | $	59,773
Options exercisable at end of period | $	$ 36,802