Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 10,695,138	$ 9,531,050	$ 7,874,050
Ceded to Other Companies	1,904,982	1,837,267	1,585,210
Assumed from Other Companies	1,213,914	1,169,084	973,597
Total net premiums written	$ 10,004,070	$ 8,862,867	$ 7,262,437
Percentage of Amount Assumed to Net	12.10%	13.20%	13.40%
Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 10,363,730	$ 9,220,683	$ 7,625,981
Ceded to Other Companies	1,799,639	1,727,854	1,490,395
Assumed from Other Companies	220,055	250,985	211,515
Total net premiums written	$ 8,784,146	$ 7,743,814	$ 6,347,101
Percentage of Amount Assumed to Net	2.50%	3.20%	3.30%
Reinsurance & Monoline Excess
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 331,408	$ 310,367	$ 248,069
Ceded to Other Companies	105,343	109,413	94,815
Assumed from Other Companies	993,859	918,099	762,082
Total net premiums written	$ 1,219,924	$ 1,119,053	$ 915,336
Percentage of Amount Assumed to Net	81.50%	82.00%	83.30%