Fair value measurements - Financial instruments disclosed, but not carried at fair value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financial instruments not carried at fair value
Debt	$ 664.8	$ 685.2
Carrying Value | Series B preference shares
Financial instruments not carried at fair value
Redeemable preference shares	205.0	223.0
2017 SEK Subordinated Notes
Financial instruments not carried at fair value
Debt	270.6	291.2
2017 SEK Subordinated Notes | Carrying Value
Financial instruments not carried at fair value
Debt	270.6	291.2
2016 SIG Senior Notes
Financial instruments not carried at fair value
Debt	394.2	394.0
2016 SIG Senior Notes | Carrying Value
Financial instruments not carried at fair value
Debt	394.2	394.0
Level 3 Inputs | Fair Value | Series B preference shares
Financial instruments not carried at fair value
Redeemable preference shares	171.4	186.4
Level 3 Inputs | 2017 SEK Subordinated Notes | Fair Value
Financial instruments not carried at fair value
Debt	268.3	294.5
Level 3 Inputs | 2016 SIG Senior Notes | Fair Value
Financial instruments not carried at fair value
Debt	$ 394.9	$ 394.5