Summary of significant accounting policies - Foreign currency and currency translation (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Summary of significant accounting policies
Foreign exchange gain (loss)	$ (0.6)	$ (0.6)	$ 0.4

Amount, before tax, of realized and unrealized gain (loss) from foreign currency transaction.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 20
-Name Accounting Standards Codification
-Section 35
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482014/830-20-35-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481956/830-20-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481926/830-20-50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481839/830-10-45-17

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration