Variable interest entities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Assets:
Fixed maturity investments	$ 1,958.9		$ 1,681.0
Short-term investments	1,132.6		1,085.2
Cash	168.4		136.3
Insurance and reinsurance premiums receivable	(789.0)		(730.1)
Funds held by ceding companies	244.1		293.9
Deferred acquisition costs	145.2		148.2
Other assets	151.5		161.4
Total assets	6,632.8		6,413.8
Liabilities
Loss and loss adjustment expense reserves	2,672.9	$ 2,515.1	2,331.5	$ 2,186.4	$ 2,023.3	$ 2,016.7
Unearned insurance and reinsurance premiums	759.4		708.0
Other liabilities	175.9		201.3
Total liabilities	4,974.3		4,548.0
VIEs
Assets:
Fixed maturity investments	3.8		3.9
Short-term investments	0.5		0.5
Cash	0.1		0.1
Total investments	4.3		4.5
Insurance and reinsurance premiums receivable	(0.5)		(0.3)
Funds held by ceding companies	2.3		3.4
Deferred acquisition costs	0.0		0.3
Other assets	0.0		0.0
Total assets	6.2		7.9
Liabilities
Loss and loss adjustment expense reserves	0.3		0.5
Unearned insurance and reinsurance premiums	0.0		0.6
Other liabilities	0.1		0.1
Total liabilities	$ 0.4		$ 1.2