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Stock-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance (in shares)	5,273,964
Shares, Granted (in shares)	1,628,022
Shares, Exercised (in shares)	(126,707)
Shares, Forfeited or expired (in shares)	(55,032)
Shares, Outstanding, Ending Balance (in shares)	6,720,247		5,273,964
Shares, Exercisable (in shares)	3,593,089
Shares, Vested and expected to vest (in shares)	6,407,781
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted- Average Exercise Price, Outstanding, Beginning Balance (in dollars per share)	$ 22.23
Weighted- Average Exercise Price, Granted (in dollars per share)	21.82
Weighted- Average Exercise Price, Exercised (in dollars per share)	2.84
Weighted- Average Exercise Price, Forfeited or expired (in dollars per share)	(21.02)
Weighted- Average Exercise Price, Outstanding, Ending Balance (in dollars per share)	22.48		$ 22.23
Weighted- Average Exercise Price, Exercisable (in dollars per share)	21.44
Weighted- Average Exercise Price, Vested and expected to vest (in dollars per share)	$ 22.41
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted- Average Remaining Contractual Term, Outstanding	7 years 4 months 24 days		6 years 9 months 18 days
Weighted- Average Remaining Contractual Term, Exercisable	5 years 10 months 24 days
Weighted- Average Remaining Contractual Term, Vested and expected to vest	7 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding, Ending Balance	$ 10,712
Aggregate Intrinsic Value, Exercisable	10,670
Aggregate Intrinsic Value, Vested and expected to vest	$ 10,707
Weighted-average grant-date fair value of options granted (in dollars per share)	$ 14.65	$ 18.75
Intrinsic value of options exercised	$ 1,600	$ 3,600
Cash received for options exercised	300	600
Fair value of shares vested	3,100	$ 3,300
Unrecognized compensation expense related to non-vested stock-options, net of related forfeiture estimates	$ 42,000
Unrecognized compensation expense recognition period	3 years 1 month 6 days

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Unrecognized cost of unvested share-based compensation awards.

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