Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) segment	Jun. 30, 2021 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,357,158	$ 1,971,624	$ 4,606,244	$ 3,821,580
Investment Income	171,574	168,187	345,086	326,764
Other	(15,845)	156,276	476,967	304,611
Total revenues	2,512,887	2,296,086	5,428,297	4,452,955
Pre-Tax Income (Loss)	222,562	299,940	954,995	596,008
Net Income (Loss) to Common Stockholders	179,322	237,238	769,960	466,763
Net investment gains	(171,555)	24,064	194,710	58,903
Net investment gains, net of tax	(134,036)	18,800	153,787	47,752
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	339	4,605	8,774	20,236
Other	147,375	124,000	265,245	229,218
Total revenues	147,714	128,604	274,019	249,454
Pre-Tax Income (Loss)	(45,521)	(90,208)	(119,393)	(154,737)
Net Income (Loss) to Common Stockholders	(36,149)	(71,209)	(95,966)	(123,817)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,727,202		3,332,181
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,070,157	1,727,202	4,032,991	3,332,181
Investment Income	114,124	116,703	251,778	221,941
Other	8,335	8,212	17,012	16,490
Total revenues	2,192,616	1,852,117	4,301,781	3,570,612
Pre-Tax Income (Loss)	347,461	291,290	729,873	548,399
Net Income (Loss) to Common Stockholders	276,133	230,313	591,685	429,022
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		244,422		489,399
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	287,001	244,422	573,253	489,399
Investment Income	57,111	46,879	84,534	84,587
Other	0	0	0	0
Total revenues	344,112	291,301	657,787	573,986
Pre-Tax Income (Loss)	92,177	74,794	149,805	143,443
Net Income (Loss) to Common Stockholders	73,374	59,334	120,454	113,806
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	93,000	90,000	189,000	178,000
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	$ 263,000	$ 221,000	$ 498,000	$ 418,000