Quarterly financial results - Revised condensed consolidated statement of comprehensive income statement (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2023	Sep. 30, 2023	Sep. 30, 2022
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)	$ 63.8	$ 61.9	$ 138.3	$ (93.6)	$ 200.2	$ 264.0	$ (363.0)
Unrealized losses from debt securities held as available for sale investments	(56.2)	(56.6)		(39.7)	(33.7)	(89.9)	(49.5)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 2.0	7.0		$ (137.7)	164.9	$ 166.9	$ (417.7)
As previously reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)		72.3	145.0		217.3
Unrealized losses from debt securities held as available for sale investments		(54.9)			(32.0)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent		19.1			183.7
Revision
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)		(10.4)	$ (6.7)		(17.1)
Unrealized losses from debt securities held as available for sale investments		(1.7)			(1.7)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent		$ (12.1)			$ (18.8)