Stock Based Compensation - Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Unrecognized compensation cost related to unvested stock options	$ 14,800		$ 14,800
Stock based compensation expense	(1,601)	$ (1,224)	(4,747)	$ (2,678)
RSU
Unrecognized compensation cost related to unvested stock options	3,600		$ 3,600
Vesting period			3 years 3 months 18 days
Stock based compensation expense	$ 300	$ 100	$ 800	$ 100
Number of Shares
Beginning Balance			51,000
Granted (in shares)			119,000	30
Settled (in shares)			(11,000)
Forfeited (in shares)			(10,000)
Ending balance	149,000		149,000
Weighted Average Grant Date Fair Value
Beginning Balance			$ 33.16
Granted (per share)			27.69
Settled (per share)			25.12
Forfeited (per share)			30.91
Ending balance	$ 29.54		$ 29.54

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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