Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,926,823	$ 2,641,914	$ 8,537,585	$ 7,686,073
Investment Income	323,756	270,944	1,015,723	739,494
Other	149,800	117,780	417,876	495,989
Total revenues	3,400,379	3,030,638	9,971,184	8,921,556
Pre-Tax Income (Loss)	474,449	419,949	1,536,192	1,253,205
Net Income (Loss) to Common Stockholders	365,634	333,586	1,180,014	984,020
Net investment gains	(8,086)	(42,426)	(40,818)	39,239
Net investment gains, net of tax	(8,043)	(33,021)	(35,008)	31,206
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	13,485	7,893	45,429	30,787
Other	148,697	151,063	430,492	429,180
Total revenues	162,182	158,956	475,921	459,967
Pre-Tax Income (Loss)	(92,111)	(58,976)	(217,912)	(253,066)
Net Income (Loss) to Common Stockholders	(72,008)	(46,226)	(169,764)	(199,040)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		2,291,917		6,650,027
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,564,490	2,291,917	7,447,828	6,650,027
Investment Income	253,234	204,030	801,573	552,917
Other	9,189	9,143	28,202	27,570
Total revenues	2,826,913	2,505,090	8,277,603	7,230,514
Pre-Tax Income (Loss)	469,421	410,909	1,437,623	1,149,886
Net Income (Loss) to Common Stockholders	363,026	326,086	1,102,363	900,434
Insurance | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	350,000	298,000	1,112,000	852,000
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		349,997		1,036,046
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	362,333	349,997	1,089,757	1,036,046
Investment Income	57,037	59,021	168,721	155,790
Other	0	0	0	0
Total revenues	419,370	409,018	1,258,478	1,191,836
Pre-Tax Income (Loss)	105,225	110,442	357,299	317,146
Net Income (Loss) to Common Stockholders	82,659	86,747	282,423	251,420
Reinsurance & Monoline Excess | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	125,000	124,000	353,000	333,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	$ 0	$ 0	$ 0	$ 0