Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning outstanding balance (in shares)	4,868
Granted (in shares)	1,581
Exercised (in shares)	(683)
Cancelled or forfeited (in shares)	(743)
Ending outstanding balance (in shares)	5,023	4,868
Exercisable (in shares)	2,733
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning outstanding balance (in dollars per share)	$ 6.28
Granted (in dollars per share)	21.15
Exercised (in dollars per share)	2.26
Cancelled or forfeited (in dollars per share)	14.95
Ending outstanding balance (in dollars per share)	10.23	$ 6.28
Exercisable (in dollars per share)	$ 5.91
Remaining contractual life, outstanding	7 years 6 months 26 days	7 years 10 months 24 days
Remaining contractual life, exercisable	6 years 9 months 22 days
Aggregate intrinsic value, outstanding	$ 3,133	$ 35,178
Aggregate intrinsic value, exercisable	$ 3,014

For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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- Definition

Weighted average price of options that were either forfeited or expired.