Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 2,081,018	$ 1,748,921	$ 5,902,598	$ 5,117,253
Investment Income	179,851	142,650	506,615	402,844
Other	163,414	148,248	468,025	266,357
Total revenues	2,424,283	2,039,819	6,877,238	5,786,454
Pre-Tax Income (Loss)	332,281	206,414	928,289	305,395
Net Income (Loss) to Common Stockholders	261,297	151,678	728,060	218,520
Net investment gains	19,501	38,978	78,404	(60,311)
Net investment gains, net of tax	15,235	30,123	62,987	(46,689)
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	12,577	14,516	32,813	45,975
Other	136,624	99,807	365,841	300,715
Total revenues	149,201	114,323	398,654	346,690
Pre-Tax Income (Loss)	(53,962)	(73,067)	(208,699)	(176,369)
Net Income (Loss) to Common Stockholders	(48,235)	(57,781)	(172,051)	(139,471)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,531,093		4,481,092
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,819,071	1,531,093	5,151,253	4,481,092
Investment Income	118,770	87,828	340,710	255,392
Other	7,289	9,463	23,780	25,953
Total revenues	1,945,130	1,628,384	5,515,743	4,762,437
Pre-Tax Income (Loss)	314,000	178,971	862,399	431,464
Net Income (Loss) to Common Stockholders	252,333	130,266	681,354	315,733
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		217,828		636,161
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	261,947	217,828	751,345	636,161
Investment Income	48,504	40,306	133,092	101,477
Other	0	0	0	0
Total revenues	310,451	258,134	884,437	737,638
Pre-Tax Income (Loss)	52,742	61,532	196,185	110,611
Net Income (Loss) to Common Stockholders	41,964	49,070	155,770	88,947
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	100,000	77,000	278,000	212,000
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	$ 220,000	$ 183,000	$ 638,000	$ 500,000