Variable interest entities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Fixed maturity investments, at fair value	$ 1,815.7		$ 1,949.2
Short-term investments	882.9		715.5
Cash	116.8		119.4
Insurance and reinsurance premiums receivable	861.3		630.6
Funds held by ceding companies	237.6		186.8
Deferred acquisition costs	158.8		141.6
Other assets	171.4		124.0
Total assets	6,419.2		6,007.7
Liabilities
Loss and loss adjustment expense reserves	2,023.3	$ 1,976.3	2,016.7	$ 1,827.1	$ 1,875.9	$ 1,898.5
Unearned insurance and reinsurance premiums	879.5		647.2
Other liabilities	186.3		150.5
Total liabilities	4,390.8		4,069.3
VIEs
Assets:
Fixed maturity investments, at fair value	3.9		4.0
Short-term investments	0.5		0.3
Cash	0.1		0.2
Total investments	4.5		4.5
Insurance and reinsurance premiums receivable	0.2		0.1
Funds held by ceding companies	4.7		3.7
Deferred acquisition costs	0.3		5.2
Other assets			0.9
Total assets	9.7		14.4
Liabilities
Loss and loss adjustment expense reserves	1.7		4.6
Unearned insurance and reinsurance premiums	0.7		3.7
Total liabilities	$ 2.4		$ 8.3