Total realized and unrealized investment gains and net investment income - Summary of net realized 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	$ 12.3	$ 46.4	$ (5.9)
Short-term investments	(0.1)	0.0	0.0
Equity securities	(0.1)	0.0	0.0
Other long-term investments	14.1	0.0	(0.1)
Net investment income on cash and cash equivalents	4.6	2.0	6.1
Net realized investment gains	30.8	48.4	0.1
Realized gains (losses) due to foreign currency	$ 4.5	$ 1.6	$ (0.7)