Total realized and unrealized investment gains and net investment income - Summary of net unrealized investment gains (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Debt securities	$ (40.2)	$ 14.9	$ 8.4
Short-term investments	1.4	0.0	0.0
Equity securities	(2.5)	0.0	0.0
Other long-term investments	11.2	0.0	0.1
Net investment income (loss) on cash and cash equivalents	(17.6)	5.9	6.7
Net unrealized investment gains (losses)	(47.7)	20.8	15.2
Debt securities, unrealized losses, excluding foreign currency	23.9
Unrealized gains (losses) due to foreign currency	$ (32.8)	$ 6.0	$ 6.9