Fair value measurements - Financial Instruments not Measured at Fair Value (Details) - Level 2 - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Fair Value \| Series B preference shares
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Series B preference shares	$ 0.0	$ 202.2
Fair Value \| Senior Notes \| 2024 Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	417.6	424.2
Fair Value \| 2017 SEK Subordinated Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	284.8	293.3
Carrying Value \| Series B preference shares
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Series B preference shares	0.0	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	396.6	396.0
Carrying Value \| 2017 SEK Subordinated Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	$ 278.9	$ 292.6

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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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 3: 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