Reinsurance (Reinsurance Financial Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reinsurance Disclosures [Abstract]
Written premiums, Direct	$ 11,676,743	$ 10,695,138	$ 9,531,050
Written premiums, Assumed	1,295,263	1,213,914	1,169,084
Written premiums, Ceded	(2,017,539)	(1,904,982)	(1,837,267)
Total net premiums written	10,954,467	10,004,070	8,862,867
Earned premiums, Direct	11,112,980	10,217,891	8,825,568
Earned premiums, Assumed	1,246,288	1,226,801	1,085,804
Earned premiums, Ceded	(1,958,581)	(1,883,263)	(1,805,341)
Net premiums earned	10,400,687	9,561,429	8,106,031
Ceded losses and loss expenses incurred	1,376,144	1,269,338	1,236,960
Ceded commission earned	471,841	477,437	449,739
Uncollectible reinsurance	$ 8,404	$ 8,064	$ 7,713