Historical Loss and LAE Development - Environmental, Asbestos and Mass Tort Exposures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Unpaid Losses and LAE at End of Year
Gross	$ 2,693,470	$ 2,446,025	$ 2,315,637	$ 2,043,555
Net	1,938,045	1,688,676	1,575,548	$ 1,435,469
Asbestos and Environmental Exposures
Loss and LAE Payments (Cumulative):
Gross	149,130	144,882	142,377
Ceded	(70,773)	(70,130)	(69,696)
Net	78,357	74,752	72,681
Unpaid Losses and LAE at End of Year
Gross	22,658	25,180	26,871
Ceded	(5,899)	(5,490)	(5,786)
Net	$ 16,759	$ 19,690	$ 21,085