Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,552,727	$ 2,357,158	$ 5,044,159	$ 4,606,244
Investment Income	245,152	171,574	468,551	345,086
Other	198,035	(15,845)	378,208	476,967
Total revenues	2,995,914	2,512,887	5,890,918	5,428,297
Pre-Tax Income (Loss)	457,184	222,562	833,256	954,995
Net Income (Loss) to Common Stockholders	356,308	179,322	650,434	769,960
Net investment gains	58,654	(171,555)	81,664	194,710
Net investment gains, net of tax	46,067	(134,036)	64,227	153,787
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	17,638	339	22,895	8,774
Other	130,528	147,375	278,117	265,245
Total revenues	148,166	147,714	301,012	274,019
Pre-Tax Income (Loss)	(93,240)	(45,521)	(194,089)	(119,393)
Net Income (Loss) to Common Stockholders	(72,315)	(36,149)	(152,814)	(95,966)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		2,070,157		4,032,991
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,246,394	2,070,157	4,428,269	4,032,991
Investment Income	187,304	114,124	353,390	251,778
Other	8,853	8,335	18,427	17,012
Total revenues	2,442,551	2,192,616	4,800,086	4,301,781
Pre-Tax Income (Loss)	386,264	347,461	738,463	729,873
Net Income (Loss) to Common Stockholders	300,152	276,133	574,056	591,685
Insurance | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	280,000	263,000	554,000	498,000
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		287,001		573,253
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	306,333	287,001	615,890	573,253
Investment Income	40,210	57,111	92,266	84,534
Other	0	0	0	0
Total revenues	346,543	344,112	708,156	657,787
Pre-Tax Income (Loss)	105,506	92,177	207,218	149,805
Net Income (Loss) to Common Stockholders	82,404	73,374	164,965	120,454
Reinsurance & Monoline Excess | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	102,000	93,000	209,000	189,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	$ 0	$ 0	$ 0	$ 0