Transactions with related parties - Transactions with Artex (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) executive	Mar. 31, 2020 GBP (£) executive	Mar. 31, 2019 USD ($)
ACGL and Subsidiaries
Related Party Transaction [Line Items]
Insurance management fees paid | $	$ 0		$ 0
Artex
Related Party Transaction [Line Items]
Related party - number of executives serving as directors | executive	2	2
Insurance management agreement termination, notice period required	12 months	12 months
Insurance management fees paid | $	$ 86,000		$ 131,000
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