Investment securities - Net investment income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Investment Income
Total investment income	$ 5.5	$ 24.8	$ 39.5	$ 76.8
Investment expenses	(3.8)	(2.0)	(9.5)	(9.5)
Net investment income	1.7	22.8	30.0	67.3
Interest on funds held	(8.1)	0.0	(7.5)	0.6
Fixed maturity investments
Net Investment Income
Total investment income	10.1	12.8	31.5	40.0
Short-term investments
Net Investment Income
Total investment income	0.6	4.1	5.2	12.0
Equity securities
Net Investment Income
Total investment income	0.8	2.7	5.5	12.5
Other Long-term Investments
Net Investment Income
Total investment income	$ (6.0)	$ 5.2	$ (2.7)	$ 12.3