Fair value measurements - Financial instruments not measured at fair value (Details) - Level 2 - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value | Series B preference shares
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Series B preference shares	$ 220.9
Fair Value | Subordinated Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	302.3
Fair Value | Senior Notes | 2016 SIG Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	412.8
Fair Value | Senior Notes | 2015 TPRUSA Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	120.5	$ 117.8
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
Carrying Value | Subordinated Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	296.3
Carrying Value | Senior Notes | 2016 SIG Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	406.0
Carrying Value | Senior Notes | 2015 TPRUSA Senior Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Long-term debt	$ 114.4	$ 114.3