Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) segment	Mar. 31, 2021 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,249,087	$ 1,849,956
Investment Income	173,512	158,577
Other	492,810	148,336
Total revenues	2,915,409	2,156,869
Pre-Tax Income (Loss)	732,433	296,068
Net Income (Loss) to Common Stockholders	590,638	229,525
Net investment gains	366,265	34,839
Net investment gains, net of tax	287,823	28,952
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	8,435	15,632
Other	117,869	105,218
Total revenues	126,304	120,850
Pre-Tax Income (Loss)	(73,872)	(64,529)
Net Income (Loss) to Common Stockholders	(59,817)	(52,606)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,604,979
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,962,835	1,604,979
Investment Income	137,654	105,237
Other	8,676	8,279
Total revenues	2,109,165	1,718,495
Pre-Tax Income (Loss)	382,412	257,109
Net Income (Loss) to Common Stockholders	315,552	198,708
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		244,977
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	286,252	244,977
Investment Income	27,423	37,708
Other	0	0
Total revenues	313,675	282,685
Pre-Tax Income (Loss)	57,628	68,649
Net Income (Loss) to Common Stockholders	47,080	54,471
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	96,000	88,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	0	0
Insurance-International Segment | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	$ 235,000	$ 196,000