Transactions with related parties - Transactions with other ACGL entities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 4,352	$ 4,409
Acquisition expenses	28,367	33,974
General and administrative expenses	$ 7,139	7,240
ACGL and Subsidiaries | AIM
Related Party Transaction [Line Items]
Investment management agreement, term	1 year
Investment management agreement termination, notice period required	45 days
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 253	262
ACGL and Subsidiaries | AUL and AUI
Related Party Transaction [Line Items]
Services agreement term	5 years
Services agreement termination, notice period required	24 months
Consolidated statement of income (loss) items:
Acquisition expenses	$ 6,563	4,948
General and administrative expenses	988	2,011
Total related party expenses	$ 7,551	$ 6,959