Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 11,676,743	$ 10,695,138	$ 9,531,050
Ceded to Other Companies	2,017,539	1,904,982	1,837,267
Assumed from Other Companies	1,295,263	1,213,914	1,169,084
Total net premiums written	$ 10,954,467	$ 10,004,070	$ 8,862,867
Percentage of Amount Assumed to Net	11.80%	12.10%	13.20%
Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 11,310,709	$ 10,363,730	$ 9,220,683
Ceded to Other Companies	1,904,017	1,799,639	1,727,854
Assumed from Other Companies	250,429	220,055	250,985
Total net premiums written	$ 9,657,121	$ 8,784,146	$ 7,743,814
Percentage of Amount Assumed to Net	2.60%	2.50%	3.20%
Reinsurance & Monoline Excess
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 366,034	$ 331,408	$ 310,367
Ceded to Other Companies	113,522	105,343	109,413
Assumed from Other Companies	1,044,834	993,859	918,099
Total net premiums written	$ 1,297,346	$ 1,219,924	$ 1,119,053
Percentage of Amount Assumed to Net	80.50%	81.50%	82.00%