Total net investment income and net realized and unrealized investment gains (losses) - Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Investments, Debt and Equity Securities [Abstract]
Debt securities, trading	$ 2.0	$ 4.3	$ 3.8	$ 9.7
Short-term investments	0.0	(0.1)	(0.2)	(1.0)
Derivative instruments	(0.3)	0.4	(0.5)	1.4
Other long-term investments	1.1	(39.6)	3.7	(38.3)
Net investment income (loss) on cash and cash equivalents	1.3	(0.3)	2.4	(2.4)
Net unrealized investment gains (losses)	$ 4.1	$ (35.3)	$ 9.2	$ (30.6)

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.