Allowance for expected credit losses (Details) $ in Millions	Sep. 30, 2020 USD ($)
Ceded Credit Risk [Line Items]
Premiums receivable & Funds held by ceding companies	$ 1,033.1
Reinsurance recoverable on unpaid and paid loss	552.9
MGU Trade receivables	17.2
Total as of June 30, 2020	1,603.2
Premiums receivable & Funds held by ceding companies, allowance for credit losses	10.6
Reinsurance recoverable on unpaid and paid loss, allowance for credit losses	3.9
MGU trade receivable, allowance for credit losses	0.4
Total allowance for credit losses	$ 14.9
Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Percent of the total reinsurance recoverables rated by either A.M. Best or S&P	0.63
AM Best, A- Rating
Ceded Credit Risk [Line Items]
Percent of the total reinsurance recoverables rated by either A.M. Best or S&P	0.67