Variable interest entities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Fixed maturity investments, at fair value	$ 1,846.4	$ 1,949.2
Short-term investments	833.6	715.5
Cash	116.0	119.4
Insurance and reinsurance premiums receivable	818.7	630.6
Funds held by ceding companies	202.8	186.8
Deferred acquisition costs	152.6	141.6
Other assets	161.1	124.0
Total assets	6,307.0	6,007.7
Liabilities
Loss and loss adjustment expense reserves	1,976.3	2,016.7	$ 1,875.9	$ 1,898.5
Unearned insurance and reinsurance premiums	860.5	647.2
Other liabilities	173.1	150.5
Total liabilities	4,291.2	4,069.3
VIEs
Assets:
Fixed maturity investments, at fair value	3.9	4.0
Short-term investments	0.4	0.3
Cash	0.2	0.2
Total investments	4.5	4.5
Insurance and reinsurance premiums receivable	1.5	0.1
Funds held by ceding companies	4.7	3.7
Deferred acquisition costs	0.5	5.2
Other assets	0.1	0.9
Total assets	11.3	14.4
Liabilities
Loss and loss adjustment expense reserves	3.0	4.6
Unearned insurance and reinsurance premiums	1.7	3.7
Other liabilities	0.1
Total liabilities	$ 4.8	$ 8.3