Stock Based Compensation - Summary of Unvested Stock Options (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Stock Based Compensation
Stock Options Outstanding - Beginning Balance	1,945
Stock Options Granted (in shares)	423
Stock option Exercised (in shares)	(81)
Stock Options Forfeited (in shares)	(147)
Stock Options Cancelled (in shares)	(1)
Stock Options Outstanding - Ending Balance	2,139	1,945
Options exercisable - Ending Balance	438
Weighted Average Exercise Price Outstanding - Beginning Balance | $ / shares	$ 23.45
Weighted Average Exercise Price - Granted | $ / shares	27.54
Weighted Average Exercise Price - Exercised | $ / shares	19.66
Weighted Average Exercise Price - Forfeited | $ / shares	21.20
Weighted Average Exercise Price - Expired | $ / shares	21.37
Weighted Average Exercise Price Outstanding - Ending Balance | $ / shares	$ 24.56	$ 23.45
Aggregate Intrinsic Value | $	$ 9,323	$ 5,527
Weighted Average Remaining Contractual Life (years)	8 years 6 months	9 years 1 month 6 days