Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 7,386,759	$ 6,973,216	$ 6,726,029
Ceded to Other Companies	1,398,719	1,269,267	1,216,455
Assumed from Other Companies	875,459	729,278	750,934
Total net premiums written	$ 6,863,499	$ 6,433,227	$ 6,260,508
Percentage of Amount Assumed to Net	12.80%	11.30%	12.00%
Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 7,180,759	$ 6,782,757	$ 6,537,777
Ceded to Other Companies	1,312,564	1,188,297	1,143,656
Assumed from Other Companies	217,814	197,445	161,394
Total net premiums written	$ 6,086,009	$ 5,791,905	$ 5,555,515
Percentage of Amount Assumed to Net	3.60%	3.40%	2.90%
Reinsurance & Monoline Excess
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 206,000	$ 190,459	$ 188,252
Ceded to Other Companies	86,155	80,970	72,799
Assumed from Other Companies	657,645	531,833	589,540
Total net premiums written	$ 777,490	$ 641,322	$ 704,993
Percentage of Amount Assumed to Net	84.60%	82.90%	83.60%