Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2023 USD ($) segment	Mar. 31, 2022 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,491,432	$ 2,249,087
Investment Income	223,398	173,512
Other	180,174	492,810
Total revenues	2,895,004	2,915,409
Pre-Tax Income (Loss)	376,071	732,433
Net Income (Loss) to Common Stockholders	294,126	590,638
Net investment gains	23,010	366,265
Net investment gains, net of tax	18,160	287,823
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	5,257	8,435
Other	147,587	117,869
Total revenues	152,844	126,304
Pre-Tax Income (Loss)	(100,850)	(73,872)
Net Income (Loss) to Common Stockholders	(80,499)	(59,817)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,962,835
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	2,181,876	1,962,835
Investment Income	166,086	137,654
Other	9,577	8,676
Total revenues	2,357,539	2,109,165
Pre-Tax Income (Loss)	352,199	382,412
Net Income (Loss) to Common Stockholders	273,904	315,552
Insurance | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	274,000	235,000
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		286,252
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	309,556	286,252
Investment Income	52,055	27,423
Other	0	0
Total revenues	361,611	313,675
Pre-Tax Income (Loss)	101,712	57,628
Net Income (Loss) to Common Stockholders	82,561	47,080
Reinsurance & Monoline Excess | Operating Segments | Non-US
Segment Reporting Information [Line Items]
Total revenues	106,000	96,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	$ 0	$ 0