Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Earned Premiums	$ 1,603,485	$ 1,581,500	$ 4,751,947	$ 4,720,244
Investment Income	186,124	142,479	514,419	426,601
Other	148,293	307,363	473,700	602,963
Total revenues	1,937,902	2,031,342	5,740,066	5,749,808
Pre-Tax Income (Loss)	209,223	226,113	649,455	571,370
Net Income (Loss) to Common Stockholders	161,920	162,054	508,392	394,505
Net investment gains	22,334	183,959	140,429	276,760
Net investment gains, net of tax	17,644	119,573	110,939	179,894
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums	0	0	0	0
Investment Income	15,979	15,027	47,306	38,251
Other	106,512	123,404	276,148	326,203
Total revenues	122,491	138,431	323,454	364,454
Pre-Tax Income (Loss)	(54,759)	(71,681)	(195,597)	(224,716)
Net Income (Loss) to Common Stockholders	(45,862)	(45,685)	(158,949)	(146,324)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,488,658	1,433,729	4,377,003	4,262,485
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	1,488,658	1,433,729	4,377,003	4,262,485
Investment Income	146,319	105,924	396,514	320,552
Other	19,447	0	57,123	0
Total revenues	1,654,424	1,539,653	4,830,640	4,583,037
Pre-Tax Income (Loss)	226,856	171,478	653,936	557,605
Net Income (Loss) to Common Stockholders	178,571	123,240	516,397	381,736
Reinsurance
Segment Reporting Information [Line Items]
Earned Premiums	114,827	147,771	374,944	457,759
Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	114,827	147,771	374,944	457,759
Investment Income	23,826	21,528	70,599	67,798
Other	0	0	0	0
Total revenues	138,653	169,299	445,543	525,557
Pre-Tax Income (Loss)	14,792	(57,643)	50,687	(38,279)
Net Income (Loss) to Common Stockholders	11,567	(35,074)	40,005	(20,801)
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	51,000	49,000	163,000	150,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums	0	0	0	0
Investment Income	0	0	0	0
Insurance-International Segment | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	$ 176,000	$ 166,000	$ 535,000	$ 513,000