Stock Based Compensation - Restricted stock units (Details) - Restricted stock units $ / shares in Units, $ in Thousands	6 Months Ended
Jun. 30, 2020 USD ($) $ / shares shares
Stock-based compensation
Beginning balance \| shares	90,006
Granted \| shares	305,794
Vested \| shares	(21,822)
Forfeited \| shares	(7,311)
Ending balance \| shares	366,667
Weighted Average Grant Date Fair Value, Beginning Balance \| $ / shares	$ 34.66
Weighted Average Grant Date Fair Value, Granted \| $ / shares	31.64
Weighted Average Grant Date Fair Value, Vested \| $ / shares	31.91
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	31.47
Weighted Average Grant Date Fair Value, Ending Balance \| $ / shares	$ 32.37
Compensation expense
Unamortized compensation expense related to unvested restricted stock units \| $	$ 10,200
Period to recognize unamortized compensation expense	2 years 3 months 18 days

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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