Fair value measurements - Financial Instruments not Measured at Fair Value (Details) - Level 2 - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Fair Value \| Series B preference shares
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Series B preference shares	$ 202.1	$ 206.0
Fair Value \| Senior Notes \| 2024 Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	421.1	411.2
Fair Value \| Subordinated Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	269.8	228.7
Carrying Value \| Series B preference shares
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Series B preference shares	200.0	200.0
Carrying Value \| Senior Notes \| 2024 Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	395.4	394.8
Carrying Value \| Subordinated Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	$ 283.0	$ 244.3

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 4: http://www.xbrl.org/2003/role/exampleRef
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Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant