Acquisition of Sirius Group - Summary of liability-classified capital instruments (Details) - USD ($) $ in Millions	10 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2021	Feb. 26, 2021	Dec. 31, 2020
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	$ 87.8	$ 87.8	$ 137.6	$ 0.0
Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	(49.8)
Series A preference shares
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	20.4	20.4	40.8
Series A preference shares | Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	(20.4)	(20.4)
Merger warrants
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	32.5	32.5	53.4
Merger warrants | Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	(20.9)	(20.9)
Private warrants
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	3.2	3.2	7.3
Private warrants | Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	(4.1)	(4.1)
Sirius Group Public Warrants
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	1.1	1.1	2.6
Sirius Group Public Warrants | Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	(1.5)	(1.5)
Upside Rights
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	0.0	0.0	6.5
Upside Rights | Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	(6.5)	(6.5)
CVRs
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Fair value	30.6	30.6	$ 27.0
CVRs | Liability-classified capital instruments
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Change in fair value	$ 3.6	$ 3.6