Total net investment income and net realized and unrealized investment gains (losses) - Net Investment Income and Total Realized and Unrealized Investment Gains (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net Investment Income [Line Items]
Investment expenses	$ (3.7)	$ (7.6)	$ (9.0)	$ (11.5)
Net investment income and net realized and unrealized investment gains (losses)	68.9	23.3	139.8	103.1
Debt securities, available for sale
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	59.7	61.6	118.5	125.3
Debt securities, trading
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	2.8	5.6	6.3	14.4
Short-term investments
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	(0.1)	2.9	1.0	7.3
Other long-term investments
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	3.3	(48.7)	5.8	(48.2)
Derivative instruments
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	(0.3)	0.4	(0.5)	1.7
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	65.4	21.8	131.1	100.5
Net investment income on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income and net realized and unrealized investment gains (losses) before other investment expenses and investment income on cash and cash equivalents	$ 7.2	$ 9.1	$ 17.7	$ 14.1