Variable interest entities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets:
Fixed maturity investments	$ 1,732.1	$ 1,681.0
Short-term investments	1,041.8	1,085.2
Cash	140.5	136.3
Insurance and reinsurance premiums receivable	964.2	730.1
Funds held by ceding companies	260.5	293.9
Deferred acquisition costs	168.2	148.2
Other assets	143.7	161.4
Total assets	6,757.6	6,413.8
Liabilities
Loss and loss adjustment expense reserves	2,519.6	2,331.5	$ 1,976.3	$ 2,016.7
Unearned insurance and reinsurance premiums	979.2	708.0
Other liabilities	200.4	201.3
Total liabilities	5,081.3	4,548.0
VIEs
Assets:
Fixed maturity investments	3.6	3.9
Short-term investments	0.5	0.5
Cash	0.1	0.1
Total investments	4.2	4.5
Insurance and reinsurance premiums receivable	(0.5)	(0.3)
Funds held by ceding companies	3.4	3.4
Deferred acquisition costs	0.1	0.3
Other assets	0.0	0.0
Total assets	7.2	7.9
Liabilities
Loss and loss adjustment expense reserves	0.4	0.5
Unearned insurance and reinsurance premiums	0.1	0.6
Other liabilities	0.1	0.1
Total liabilities	$ 0.6	$ 1.2