Variable interest entities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets:
Fixed maturity investments	$ 1,904.7		$ 1,681.0
Short-term investments	1,038.8		1,085.2
Cash	186.7		136.3
Insurance and reinsurance premiums receivable	871.7		730.1
Funds held by ceding companies	254.7		293.9
Deferred acquisition costs	159.1		148.2
Other assets	153.8		161.4
Total assets	6,664.0		6,413.8
Liabilities
Loss and loss adjustment expense reserves	2,515.1	$ 2,519.6	2,331.5	$ 2,023.3	$ 1,976.3	$ 2,016.7
Unearned insurance and reinsurance premiums	874.5		708.0
Other liabilities	181.7		201.3
Total liabilities	4,935.6		4,548.0
VIEs
Assets:
Fixed maturity investments	3.7		3.9
Short-term investments	0.5		0.5
Cash	1.0		0.1
Total investments	5.2		4.5
Insurance and reinsurance premiums receivable	(0.6)		(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	6.9		7.9
Liabilities
Loss and loss adjustment expense reserves	0.4		0.5
Unearned insurance and reinsurance premiums	0.0		0.6
Other liabilities	0.1		0.1
Total liabilities	$ 0.5		$ 1.2