Fair value measurements - Financial instruments disclosed, but not carried at fair value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2011
Financial instruments not carried at fair value
Debt	$ 696.8	$ 723.2
Carrying Value | Series B preference shares
Financial instruments not carried at fair value
Redeemable preference shares	232.2
Carrying Value | Series A preference shares
Financial instruments not carried at fair value
Redeemable preference shares		106.1
2017 SEK Subordinated Notes
Financial instruments not carried at fair value
Debt	303.6	330.7
2017 SEK Subordinated Notes | Carrying Value
Financial instruments not carried at fair value
Debt	303.6	330.7
2016 SIG Senior Notes
Financial instruments not carried at fair value
Debt	393.2	392.5
2016 SIG Senior Notes | Carrying Value
Financial instruments not carried at fair value
Debt	393.2	392.5
Old Lyme Note
Financial instruments not carried at fair value
Debt			$ 2.1
Level 3 | Fair Value | Series B preference shares
Financial instruments not carried at fair value
Redeemable preference shares	191.7
Level 3 | Fair Value | Series A preference shares
Financial instruments not carried at fair value
Redeemable preference shares		90.0
Level 3 | 2017 SEK Subordinated Notes | Fair Value
Financial instruments not carried at fair value
Debt	309.5	341.4
Level 3 | 2016 SIG Senior Notes | Fair Value
Financial instruments not carried at fair value
Debt	$ 347.6	$ 392.3