Industry Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									2
Earned Premiums									$ 6,633,288	$ 6,371,505	$ 6,311,419
Investment Income									645,614	674,235	575,788
Net investment gains									120,703	154,488	335,858
Other Revenues									623,294	645,911	797,557
Total revenues	$ 1,976,074	$ 1,965,716	$ 2,023,384	$ 1,937,022	$ 1,951,586	$ 1,937,902	$ 1,910,916	$ 1,891,247	7,902,196	7,691,651	7,684,764
Pre-Tax Income (Loss)									852,920	812,094	772,770
Net investment gains									95,355	122,046	218,308
Net income to common stockholders	$ 119,306	$ 165,208	$ 216,709	$ 180,722	$ 132,357	$ 161,920	$ 180,075	$ 166,397	681,944	640,749	549,094
Insurance
Segment Reporting Information [Line Items]
Earned Premiums									5,919,819	5,702,073	5,549,403
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums									713,469	669,432	762,016
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Earned Premiums									5,919,819	5,702,073	5,549,403
Investment Income									429,405	433,490	366,862
Other Revenues									47,850	72,727	86,865
Total revenues									6,397,074	6,208,290	6,003,130
Pre-Tax Income (Loss)									814,862	717,154	623,746
Net income to common stockholders									650,510	571,381	437,953
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums									713,469	669,432	762,016
Investment Income									164,082	179,534	160,462
Other Revenues									0	0	0
Total revenues									877,551	848,966	922,478
Pre-Tax Income (Loss)									189,188	201,001	117,131
Net income to common stockholders									152,046	160,791	90,358
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Investment Income									52,127	61,211	48,464
Other Revenues									454,741	418,696	374,834
Total revenues									506,868	479,907	423,298
Pre-Tax Income (Loss)									(271,833)	(260,549)	(303,965)
Net income to common stockholders									$ (215,967)	$ (213,469)	$ (197,525)