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Fair value measurements - Financial instruments disclosed, but not carried at fair value (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Financial instruments not carried at fair value    
Debt $ 684.9 $ 685.2
Carrying Value | Series B preference shares    
Financial instruments not carried at fair value    
Redeemable preference shares 206.2 223.0
2017 SEK Subordinated Notes    
Financial instruments not carried at fair value    
Debt 290.5 291.2
2017 SEK Subordinated Notes | Carrying Value    
Financial instruments not carried at fair value    
Debt 290.5 291.2
2016 SIG Senior Notes    
Financial instruments not carried at fair value    
Debt 394.4 394.0
2016 SIG Senior Notes | Carrying Value    
Financial instruments not carried at fair value    
Debt 394.4 394.0
Level 3 Inputs | Fair Value | Series B preference shares    
Financial instruments not carried at fair value    
Redeemable preference shares 179.5 186.4
Level 3 Inputs | 2017 SEK Subordinated Notes | Fair Value    
Financial instruments not carried at fair value    
Debt 268.7 294.5
Level 3 Inputs | 2016 SIG Senior Notes | Fair Value    
Financial instruments not carried at fair value    
Debt $ 367.4 $ 394.5