Total realized and unrealized investment gains (losses) and net investment income - Summary of net realized investment gains (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Debt securities, available for sale	$ 2.7	$ 0.0	$ 0.0
Debt securities, trading	(66.7)	12.3	46.4
Short-term investments	(2.9)	(0.1)	0.0
Equity securities	(2.3)	(0.1)	0.0
Other long-term investments	2.9	14.1	0.0
Net investment income (loss) on cash and cash equivalents	(9.8)	4.6	2.0
Net realized gains (losses) on investments	(76.1)	30.8	48.4
Realized gains (losses) due to foreign currency	$ (25.2)	$ 4.5	$ 1.6