Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Earned Premiums (1)	$ 1,676,810	$ 1,603,485	$ 4,916,507	$ 4,751,947
Investment Income	161,692	186,124	508,279	514,419
Other	127,214	148,293	501,336	473,700
Total revenues	1,965,716	1,937,902	5,926,122	5,740,066
Pre-Tax Income (Loss)	202,982	209,223	706,157	649,455
Net Income (Loss) to Common Stockholders	165,208	161,920	562,638	508,392
Net investment gains	1,465	22,334	143,691	140,429
Net investment gains, net of tax	1,158	17,644	113,516	110,939
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	18,296	15,979	48,416	47,306
Other	114,138	106,512	319,103	276,148
Total revenues	132,434	122,491	367,519	323,454
Pre-Tax Income (Loss)	(47,736)	(54,759)	(194,664)	(195,597)
Net Income (Loss) to Common Stockholders	(37,184)	(45,862)	(154,736)	(158,949)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,493,854	1,447,628	4,396,071	4,254,497
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,493,854	1,447,628	4,396,071	4,254,497
Investment Income	99,628	124,774	332,072	332,814
Other	11,611	19,447	38,542	57,123
Total revenues	1,605,093	1,591,849	4,766,685	4,644,434
Pre-Tax Income (Loss)	202,390	193,588	612,777	555,697
Net Income (Loss) to Common Stockholders	162,994	152,396	488,228	438,822
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)	182,956	155,857	520,436	497,450
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	182,956	155,857	520,436	497,450
Investment Income	43,768	45,371	127,791	134,299
Other	0	0	0	0
Total revenues	226,724	201,228	648,227	631,749
Pre-Tax Income (Loss)	46,863	48,060	144,353	148,926
Net Income (Loss) to Common Stockholders	38,240	37,742	115,630	117,580
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	65,000	51,000	186,000	163,000
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	$ 185,000	$ 176,000	$ 535,000	$ 535,000