Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Earned Premiums (1)	$ 1,691,418	$ 1,592,856
Investment Income	174,763	158,254
Other	(55,571)	185,912
Total revenues	1,810,610	1,937,022
Pre-Tax (Loss) Income	(6,468)	229,439
Net (Loss) Income to Common Stockholders	(4,418)	180,722
Net investment gains	(177,174)	68,653
Net investment gains, net of tax	(139,967)	54,236
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0
Investment Income	13,595	18,639
Other	113,130	104,015
Total revenues	126,725	122,654
Pre-Tax (Loss) Income	(41,755)	(68,585)
Net (Loss) Income to Common Stockholders	(33,755)	(55,032)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)		1,427,034
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,484,955	1,427,034
Investment Income	123,458	100,041
Other	8,473	13,244
Total revenues	1,616,886	1,540,319
Pre-Tax (Loss) Income	175,947	184,516
Net (Loss) Income to Common Stockholders	140,117	145,993
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)		165,822
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	206,463	165,822
Investment Income	37,710	39,574
Other	0	0
Total revenues	244,173	205,396
Pre-Tax (Loss) Income	36,514	44,855
Net (Loss) Income to Common Stockholders	29,187	35,525
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	69,000	59,000
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	$ 168,000	$ 170,000