Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2025 USD ($) segment	Mar. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 3,012,381	$ 2,764,347
Investment Income	360,292	319,839
Other	174,726	172,587
Total revenues	3,547,399	3,256,773
Net Income (Loss) to Common Stockholders	417,571	442,471
Net investment gains	16,355	25,780
Net investment gains, net of tax	12,827	19,147
Income before income taxes	538,606	574,071
Policyholder Benefits and Claims Incurred, Net	1,900,792	1,663,778
Policy Acquisition And Insurance Operating Expenses	838,246	791,532
Segment Reporting, Other Segment Item, Amount	269,755	227,392
Total	3,008,793	2,682,702
Net investment gains	16,355	25,780
Corporate Reconciling Items And Eliminations
Segment Reporting Information [Line Items]
Investment Income	2,614	21,850
Other	148,419	137,384
Total revenues	151,033	159,234
Net Income (Loss) to Common Stockholders	(84,221)	(43,893)
Income before income taxes	(107,634)	(57,482)
Segment Reporting, Other Segment Item, Amount	258,667	216,716
Total	258,667	216,716
Segment Reporting, Reconciling Item, Corporate Nonsegment
Segment Reporting Information [Line Items]
Income before income taxes	16,355	25,780
Net investment gains	16,355
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		2,398,768
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,642,507	2,398,768
Investment Income	291,248	244,778
Other	9,952	9,423
Total revenues	2,943,707	2,652,969
Net Income (Loss) to Common Stockholders	393,122	365,091
Income before income taxes	509,505	478,149
Policyholder Benefits and Claims Incurred, Net	1,687,453	1,481,552
Policy Acquisition And Insurance Operating Expenses	735,661	682,592
Segment Reporting, Other Segment Item, Amount	11,088	10,676
Total	2,434,202	2,174,820
Insurance | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	333,000	393,000
Reinsurance and Monoline Excess Segment
Segment Reporting Information [Line Items]
Earned Premiums (1)		365,579
Reinsurance and Monoline Excess Segment | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	369,874	365,579
Investment Income	66,430	53,211
Other	0	0
Total revenues	436,304	418,790
Net Income (Loss) to Common Stockholders	95,843	102,126
Income before income taxes	120,380	127,624
Policyholder Benefits and Claims Incurred, Net	213,339	182,226
Policy Acquisition And Insurance Operating Expenses	102,585	108,940
Total	315,924	291,166
Reinsurance and Monoline Excess Segment | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	$ 130,000	$ 111,000