Historical Loss and LAE Development - Environmental, Asbestos and Mass Tort Exposures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Unpaid losses and LAE at beginning of year
Gross	$ 2,446,025	$ 2,315,637	$ 2,043,555	$ 1,750,049
Net	1,688,676	1,575,548	1,435,469	$ 1,306,320
Asbestos and Environmental Exposures
Loss and LAE Payments (Cumulative):
Gross	144,882	142,377	141,768
Ceded	(70,130)	(69,696)	(69,576)
Net	74,752	72,681	72,192
Unpaid losses and LAE at beginning of year
Gross	25,180	26,871	25,747
Ceded	(5,490)	(5,786)	(5,718)
Net	$ 19,690	$ 21,085	$ 20,029