Transactions with related parties - Transactions with Artex (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) executive	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) executive	Jun. 30, 2020 GBP (£) executive	Jun. 30, 2019 USD ($)
ACGL and Subsidiaries
Related Party Transaction [Line Items]
Insurance management fees paid | $	$ 0	$ 0	$ 0		$ 0
Artex
Related Party Transaction [Line Items]
Related party - number of executives serving as directors | executive	2		2	2
Insurance management agreement termination, notice period required			12 months	12 months
Insurance management fees paid | $	$ 166,000	$ 45,000	$ 252,000		$ 176,000
Artex | Minimum
Related Party Transaction [Line Items]
Insurance management agreed service fees | £				£ 150,000
Artex | Maximum
Related Party Transaction [Line Items]
Insurance management agreed service fees | £				£ 400,000