Stock-Based Compensation Plans - Stock compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Stock-based compensation plans
Stock compensation expense	$ 2,762	$ 2,117
Employee Stock Purchase Plan
Stock-based compensation plans
Stock compensation expense	78	32
2007 Stock Incentive Plan
Stock-based compensation plans
Stock compensation expense	685	1,025
2018 Stock Incentive Plan
Stock-based compensation plans
Stock compensation expense	$ 1,999	$ 1,060

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section S99
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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