Net investment income - Summary of net unrealized investment gains (losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Debt securities	$ (15.2)	$ (1.8)	$ (23.5)	$ 14.2
Short-term investments	(6.8)	0.0	(10.7)	0.0
Equity securities	(1.4)	0.0	(1.4)	0.0
Other long-term investments	4.9	0.0	53.5	0.0
Net investment income (loss) on cash and cash equivalents	1.1	5.3	2.8	(4.0)
Net unrealized investment gains (losses)	$ (17.4)	$ 3.5	$ 20.7	$ 10.2