Fair value disclosures - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 27,674,000	$ 27,669,000
Fair value, measurements, recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of liabilities	0
Fair value, measurements, recurring \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	27,674,000	27,669,000
Fair value, measurements, recurring \| Mutual funds held in Rabbi Trust
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	847,000	810,000
Fair value, measurements, recurring \| Level 1 \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	27,674,000	27,669,000
Fair value, measurements, recurring \| Level 1 \| Mutual funds held in Rabbi Trust
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	847,000	810,000
Fair value, measurements, recurring \| Level 2 \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Fair value, measurements, recurring \| Level 2 \| Mutual funds held in Rabbi Trust
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Fair value, measurements, recurring \| Level 3 \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Fair value, measurements, recurring \| Level 3 \| Mutual funds held in Rabbi Trust
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Fair value, measurements, nonrecurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of liabilities	0	0
Fair value of assets	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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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.

+ References

No definition available.

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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