Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 9,531,050	$ 7,874,050	$ 7,386,759
Ceded to Other Companies	1,837,267	1,585,210	1,398,719
Assumed from Other Companies	1,169,084	973,597	875,459
Total net premiums written	$ 8,862,867	$ 7,262,437	$ 6,863,499
Percentage of Amount Assumed to Net	13.20%	13.40%	12.80%
Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 9,220,683	$ 7,625,981	$ 7,180,759
Ceded to Other Companies	1,727,854	1,490,395	1,312,564
Assumed from Other Companies	250,985	211,515	217,814
Total net premiums written	$ 7,743,814	$ 6,347,101	$ 6,086,009
Percentage of Amount Assumed to Net	3.20%	3.30%	3.60%
Reinsurance & Monoline Excess
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 310,367	$ 248,069	$ 206,000
Ceded to Other Companies	109,413	94,815	86,155
Assumed from Other Companies	918,099	762,082	657,645
Total net premiums written	$ 1,119,053	$ 915,336	$ 777,490
Percentage of Amount Assumed to Net	82.00%	83.30%	84.60%