Summary of Significant Accounting Policies - Effects of Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Premiums Written
Direct	$ 4,558,500	$ 4,048,500	$ 3,907,500
Net	4,460,000	3,980,000	3,860,000
Premiums Earned
Direct	4,368,342	4,022,611	3,793,664
Ceded	(109,445)	(80,950)	(65,017)
Assumed	15,481	10,821	13,301
Net	4,274,378	3,952,482	3,741,948
Property & Casualty
Premiums Written
Direct	4,531,942	4,018,057	3,877,476
Ceded	(109,644)	(81,256)	(65,521)
Assumed	15,353	10,743	13,361
Net	4,437,651	3,947,544	3,825,316
Premiums Earned
Direct	4,339,333	3,993,771	3,764,626
Ceded	(109,445)	(80,950)	(65,017)
Assumed	15,481	10,821	13,301
Net	$ 4,245,369	$ 3,923,642	$ 3,712,910