Commitments and contingencies: Current and potential litigation (Details) $ in Millions	6 Months Ended
Jun. 30, 2022 USD ($)
Commitments and contingencies
Estimate of possible loss in excess of accrual	$ 3.7
CIMA
Commitments and contingencies
Alleged unpaid fees	0.6

Presents an assertion of a fact by a plaintiff in a pleading or complaint, which the plaintiff claims it will prove upon presentation of evidence at the proceeding.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14435-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14557-108349

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration