Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Earned Premiums (1)	$ 1,676,915	$ 1,646,841	$ 3,368,333	$ 3,239,697
Investment Income	85,431	188,333	260,194	346,587
Other	173,680	188,210	118,108	374,122
Total revenues	1,936,026	2,023,384	3,746,635	3,960,406
Pre-Tax Income (Loss)	105,449	273,736	98,982	503,175
Net Income (Loss) to Common Stockholders	71,260	216,709	66,842	397,431
Net investment gains	77,885	73,574	(99,289)	142,226
Net investment gains, net of tax	59,787	58,123	(76,813)	112,359
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	17,865	11,481	31,459	30,120
Other	87,777	100,949	200,907	204,965
Total revenues	105,642	112,430	232,366	235,085
Pre-Tax Income (Loss)	(61,548)	(78,344)	(103,302)	(146,928)
Net Income (Loss) to Common Stockholders	(47,823)	(62,519)	(81,689)	(117,551)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,475,184		2,902,218
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,465,044	1,475,184	2,949,999	2,902,218
Investment Income	44,105	132,403	167,564	232,444
Other	8,018	13,687	16,490	26,931
Total revenues	1,517,167	1,621,274	3,134,053	3,161,593
Pre-Tax Income (Loss)	76,546	225,871	252,493	410,387
Net Income (Loss) to Common Stockholders	48,607	179,241	185,467	325,234
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		171,657		337,479
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	211,871	171,657	418,334	337,479
Investment Income	23,461	44,449	61,171	84,023
Other	0	0	0	0
Total revenues	235,332	216,106	479,505	421,502
Pre-Tax Income (Loss)	12,566	52,635	49,080	97,490
Net Income (Loss) to Common Stockholders	10,689	41,864	39,877	77,389
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	65,000	62,000	134,000	121,000
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	$ 149,000	$ 179,000	$ 317,000	$ 349,000