Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Mar. 31, 2023 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,764,347	$ 2,491,432
Investment Income	319,839	223,398
Other	172,587	180,174
Total revenues	3,256,773	2,895,004
Pre-Tax Income (Loss)	574,071	376,071
Net Income (Loss) to Common Stockholders	442,471	294,126
Net investment gains	25,780	23,010
Net investment gains, net of tax	19,147	18,160
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	21,850	5,257
Other	137,384	147,587
Total revenues	159,234	152,844
Pre-Tax Income (Loss)	(57,482)	(100,850)
Net Income (Loss) to Common Stockholders	(43,893)	(80,499)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		2,142,924
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,398,768	2,142,924
Investment Income	244,778	163,558
Other	9,423	9,577
Total revenues	2,652,969	2,316,059
Pre-Tax Income (Loss)	478,149	352,626
Net Income (Loss) to Common Stockholders	365,091	274,108
Insurance | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	393,000	274,000
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		348,508
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	365,579	348,508
Investment Income	53,211	54,583
Other	0	0
Total revenues	418,790	403,091
Pre-Tax Income (Loss)	127,624	101,285
Net Income (Loss) to Common Stockholders	102,126	82,357
Reinsurance & Monoline Excess | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	111,000	106,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	$ 0	$ 0