Transactions with related parties (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Transactions with related parties
Gross written premiums	$ 487.1	$ 505.0	$ 1,109.4	$ 1,120.2
Receivables due from affiliates	23.2		23.2		$ 14.3
Payables due to affiliates	0.0		0.0		$ 0.0
Insurance and MGU affiliates, or their subsidiaries
Transactions with related parties
Gross written premiums	$ 24.9	$ 18.9	$ 49.0	$ 38.2