Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,441,961	$ 2,081,018	$ 7,048,205	$ 5,902,598
Investment Income	202,816	179,851	547,902	506,615
Other	79,623	163,414	556,590	468,025
Total revenues	2,724,400	2,424,283	8,152,697	6,877,238
Pre-Tax Income (Loss)	286,265	332,281	1,241,260	928,289
Net Income (Loss) to Common Stockholders	228,879	261,297	998,839	728,060
Net investment gains	(67,410)	19,501	127,299	78,404
Net investment gains, net of tax	(52,954)	15,235	100,833	62,987
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	13,063	12,577	21,837	32,813
Other	138,692	136,624	403,937	365,841
Total revenues	151,755	149,201	425,774	398,654
Pre-Tax Income (Loss)	(28,198)	(53,962)	(147,590)	(208,699)
Net Income (Loss) to Common Stockholders	(23,563)	(48,235)	(119,530)	(172,051)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,819,071		5,151,253
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,129,014	1,819,071	6,162,005	5,151,253
Investment Income	134,519	118,770	386,297	340,710
Other	8,341	7,289	25,354	23,780
Total revenues	2,271,874	1,945,130	6,573,656	5,515,743
Pre-Tax Income (Loss)	322,312	314,000	1,052,185	862,399
Net Income (Loss) to Common Stockholders	258,521	252,333	850,207	681,354
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		261,947		751,345
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	312,947	261,947	886,200	751,345
Investment Income	55,234	48,504	139,768	133,092
Other	0	0	0	0
Total revenues	368,181	310,451	1,025,968	884,437
Pre-Tax Income (Loss)	59,561	52,742	209,366	196,185
Net Income (Loss) to Common Stockholders	46,875	41,964	167,329	155,770
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	117,000	100,000	307,000	278,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	0	0	0	0
Insurance-International Segment | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	$ 252,000	$ 220,000	$ 750,000	$ 638,000