Transactions with related parties - Transactions with other ACGL entities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 4,262	$ 4,570	$ 8,614	$ 8,979
Acquisition expenses	29,486	35,417	57,853	69,391
General and administrative expenses	7,841	9,751	$ 14,980	16,991
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	223	273	$ 476	535
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	6,942	5,861	$ 13,505	10,809
General and administrative expenses	1,148	2,008	2,136	4,019
Total related party expenses	$ 8,090	$ 7,869	$ 15,641	$ 14,828