Industry Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Earned Premiums	$ 8,106,031	$ 6,930,843	$ 6,633,288
Investment Income	671,618	583,821	645,614
Net investment gains	90,632	103,000	120,703
Other Revenues	677,817	584,261	623,294
Total revenues	9,455,466	8,098,925	7,902,196
Pre-Tax Income (Loss)	1,282,905	704,802	852,920
Net investment gains	72,922	93,181	95,355
Net income to common stockholders	1,022,490	530,670	681,944
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	7,077,708	6,067,669	5,919,819
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	1,028,323	863,174	713,469
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Earned Premiums	7,077,708	6,067,669	5,919,819
Investment Income	468,821	375,554	429,405
Other Revenues	32,063	35,611	47,850
Total revenues	7,578,592	6,478,834	6,397,074
Pre-Tax Income (Loss)	1,219,798	668,012	814,862
Net income to common stockholders	976,184	487,125	650,510
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	1,028,323	863,174	713,469
Investment Income	175,324	146,029	164,082
Other Revenues	0	0	0
Total revenues	1,203,647	1,009,203	877,551
Pre-Tax Income (Loss)	270,563	205,587	189,188
Net income to common stockholders	215,439	164,655	152,046
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Investment Income	27,473	62,238	52,127
Other Revenues	555,122	445,650	454,741
Total revenues	582,595	507,888	506,868
Pre-Tax Income (Loss)	(298,088)	(271,797)	(271,833)
Net income to common stockholders	$ (242,055)	$ (214,291)	$ (215,967)