Industry Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Earned Premiums	$ 6,930,843	$ 6,633,288	$ 6,371,505
Investment Income	583,821	645,614	674,235
Net investment gains	103,000	120,703	154,488
Other Revenues	584,261	623,294	645,911
Total revenues	8,098,925	7,902,196	7,691,651
Pre-Tax Income (Loss)	704,802	852,920	812,094
Net investment gains	93,181	95,355	122,046
Net income to common stockholders	530,670	681,944	640,749
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	6,067,669	5,919,819	5,702,073
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	863,174	713,469	669,432
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Earned Premiums	6,067,669	5,919,819	5,702,073
Investment Income	375,554	429,405	433,490
Other Revenues	35,611	47,850	72,727
Total revenues	6,478,834	6,397,074	6,208,290
Pre-Tax Income (Loss)	668,012	814,862	717,154
Net income to common stockholders	487,125	650,510	571,381
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	863,174	713,469	669,432
Investment Income	146,029	164,082	179,534
Other Revenues	0	0	0
Total revenues	1,009,203	877,551	848,966
Pre-Tax Income (Loss)	205,587	189,188	201,001
Net income to common stockholders	164,655	152,046	160,791
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Investment Income	62,238	52,127	61,211
Other Revenues	445,650	454,741	418,696
Total revenues	507,888	506,868	479,907
Pre-Tax Income (Loss)	(271,797)	(271,833)	(260,549)
Net income to common stockholders	$ (214,291)	$ (215,967)	$ (213,469)