Industry Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) segment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Earned Premiums	$ 9,561,429	$ 8,106,031	$ 6,930,843
Investment Income	779,185	671,618	583,821
Net investment gains	202,397	90,632	103,000
Other Revenues	825,885	677,817	584,261
Total revenues	11,166,499	9,455,466	8,098,925
Pre-Tax Income (Loss)	1,719,681	1,282,905	704,802
Net investment gains	159,727	72,922	93,181
Net income to common stockholders	1,381,062	1,022,490	530,670
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	8,369,062	7,077,708	6,067,669
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	1,192,367	1,028,323	863,174
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Earned Premiums	8,369,062	7,077,708	6,067,669
Investment Income	550,084	468,821	375,554
Other Revenues	33,347	32,063	35,611
Total revenues	8,952,493	7,578,592	6,478,834
Pre-Tax Income (Loss)	1,455,658	1,219,798	668,012
Net income to common stockholders	1,173,425	976,184	487,125
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums	1,192,367	1,028,323	863,174
Investment Income	194,272	175,324	146,029
Other Revenues	0	0	0
Total revenues	1,386,639	1,203,647	1,009,203
Pre-Tax Income (Loss)	316,527	270,563	205,587
Net income to common stockholders	251,386	215,439	164,655
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Investment Income	34,829	27,473	62,238
Other Revenues	590,141	555,122	445,650
Total revenues	624,970	582,595	507,888
Pre-Tax Income (Loss)	(254,901)	(298,088)	(271,797)
Net income to common stockholders	$ (203,476)	$ (242,055)	$ (214,291)