Premiums and Reinsurance Related Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Written premiums:
Written premiums, direct	$ 2,985,563	$ 2,757,739	$ 5,724,317	$ 5,300,075
Written premiums, assumed	351,210	294,662	661,773	612,163
Written premiums, ceded	(525,258)	(466,766)	(999,751)	(913,349)
Total net premiums written	2,811,515	2,585,635	5,386,339	4,998,889
Earned premiums:
Earned premiums, direct	2,725,650	2,511,579	5,392,712	4,916,177
Earned premiums, assumed	302,431	292,547	597,661	584,920
Earned premiums, ceded	(475,354)	(446,968)	(946,214)	(894,853)
Net premiums earned	2,552,727	2,357,158	5,044,159	4,606,244
Ceded losses and loss expenses incurred	333,795	305,196	649,271	548,490
Ceded commissions earned	117,523	116,991	235,941	234,436
Cumulative effect adjustment resulting from changes in accounting principles	$ 9,365	$ 7,744	$ 9,365	$ 7,744	$ 8,703	$ 8,064	$ 7,655	$ 7,713