Industry Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Earned Premiums	$ 11,548,485	$ 10,400,687	$ 9,561,429
Investment Income	1,333,161	1,052,835	779,185
Net investment gains	117,708	47,042	202,397
Other Revenues	757,106	689,416	825,885
Total revenues	13,638,752	12,142,938	11,166,499
Losses and Loss Expenses	7,131,595	6,372,142	5,861,750
Policy Acquisition and Insurance Operating Expenses	3,294,902	2,954,686	2,673,903
Other	947,762	1,061,707	911,165
Total operating costs and expenses	11,374,259	10,388,535	9,446,818
Pre-Tax Income (Loss)	2,264,493	1,754,403	1,719,681
Net income to common stockholders	1,756,115	1,381,359	1,381,062
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	10,086,308	9,007,376	8,171,828
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	1,462,177	1,393,311	1,389,601
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Earned Premiums	10,086,308	9,007,376	8,171,828
Investment Income	1,057,738	783,660	543,844
Other Revenues	37,455	36,830	33,347
Total revenues	11,181,501	9,827,866	8,749,019
Losses and Loss Expenses	6,332,490	5,615,526	5,013,614
Policy Acquisition and Insurance Operating Expenses	2,863,697	2,545,310	2,268,649
Other	43,231	37,112	31,294
Total operating costs and expenses	9,239,418	8,197,948	7,313,557
Pre-Tax Income (Loss)	1,942,083	1,629,918	1,435,462
Net income to common stockholders	1,503,875	1,283,281	1,153,231
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	1,462,177	1,393,311	1,389,601
Investment Income	234,728	221,966	200,512
Other Revenues	0	0	0
Total revenues	1,696,905	1,615,277	1,590,113
Losses and Loss Expenses	799,105	756,616	848,136
Policy Acquisition and Insurance Operating Expenses	431,205	409,376	405,254
Total operating costs and expenses	1,230,310	1,165,992	1,253,390
Pre-Tax Income (Loss)	466,595	449,285	336,723
Net income to common stockholders	367,569	355,155	271,580
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Investment Income	40,695	47,209	34,829
Other Revenues	601,943	605,544	590,141
Total revenues	642,638	652,753	624,970
Other	904,531	1,024,595	879,871
Total operating costs and expenses	904,531	1,024,595	879,871
Pre-Tax Income (Loss)	(261,893)	(371,842)	(254,901)
Net income to common stockholders	(203,832)	(293,869)	(203,476)
Net investment gains
Segment Reporting Information [Line Items]
Net investment gains	117,708	47,042	202,397
Pre-Tax Income (Loss)	117,708	47,042	202,397
Net income to common stockholders	$ 88,503	$ 36,792	$ 159,727