Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 7,874,050	$ 7,386,759	$ 6,973,216
Ceded to Other Companies	1,585,210	1,398,719	1,269,267
Assumed from Other Companies	973,597	875,459	729,278
Total net premiums written	$ 7,262,437	$ 6,863,499	$ 6,433,227
Percentage of Amount Assumed to Net	13.40%	12.80%	11.30%
Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 7,625,981	$ 7,180,759	$ 6,782,757
Ceded to Other Companies	1,490,395	1,312,564	1,188,297
Assumed from Other Companies	211,515	217,814	197,445
Total net premiums written	$ 6,347,101	$ 6,086,009	$ 5,791,905
Percentage of Amount Assumed to Net	3.30%	3.60%	3.40%
Reinsurance & Monoline Excess
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 248,069	$ 206,000	$ 190,459
Ceded to Other Companies	94,815	86,155	80,970
Assumed from Other Companies	762,082	657,645	531,833
Total net premiums written	$ 915,336	$ 777,490	$ 641,322
Percentage of Amount Assumed to Net	83.30%	84.60%	82.90%