Total realized and unrealized investment gains and net investment income - Summary of gains (losses) attributable to unrealized investment gains (losses) on level 3 investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net Investment Income [Line Items]
Other long-term investments	$ 11.2	$ 0.0	$ 0.1
Net unrealized investment gains (losses)	(47.7)	20.8	15.2
Level 3
Net Investment Income [Line Items]
Other long-term investments	(5.7)	0.0	0.0
Net unrealized investment gains (losses)	$ (5.7)	$ 0.0	$ 0.0