Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 1,748,921	$ 1,676,810	$ 5,117,253	$ 4,916,507
Investment Income	142,650	161,692	402,844	508,279
Other	148,248	127,214	266,357	501,336
Total revenues	2,039,819	1,965,716	5,786,454	5,926,122
Pre-Tax Income (Loss)	206,414	202,982	305,395	706,157
Net Income (Loss) to Common Stockholders	151,678	165,208	218,520	562,638
Net investment gains	38,978	1,465	(60,311)	143,691
Net investment gains, net of tax	30,123	1,158	(46,689)	113,516
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	14,516	18,296	45,975	48,416
Other	99,807	114,138	300,715	319,103
Total revenues	114,323	132,434	346,690	367,519
Pre-Tax Income (Loss)	(73,067)	(47,736)	(176,369)	(194,664)
Net Income (Loss) to Common Stockholders	(57,781)	(37,184)	(139,471)	(154,736)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,493,854		4,396,071
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,531,093	1,493,854	4,481,092	4,396,071
Investment Income	87,828	99,628	255,392	332,072
Other	9,463	11,611	25,953	38,542
Total revenues	1,628,384	1,605,093	4,762,437	4,766,685
Pre-Tax Income (Loss)	178,971	202,390	431,464	612,777
Net Income (Loss) to Common Stockholders	130,266	162,994	315,733	488,228
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		182,956		520,436
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	217,828	182,956	636,161	520,436
Investment Income	40,306	43,768	101,477	127,791
Other	0	0	0	0
Total revenues	258,134	226,724	737,638	648,227
Pre-Tax Income (Loss)	61,532	46,863	110,611	144,353
Net Income (Loss) to Common Stockholders	49,070	38,240	88,947	115,630
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	77,000	65,000	212,000	186,000
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	$ 183,000	$ 185,000	$ 500,000	$ 535,000