Segment Information (Income From Continuing Operations, Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement [Line Items]
Net investment income earned	$ 388,650	$ 288,155	$ 326,589
Net realized and unrealized investment gains (losses)	(3,552)	(114,808)	17,599
Total investment revenues, before federal income tax	385,098	173,347	344,188
Tax on investment income	78,369	31,846	67,284
Total investment income, after federal income tax	306,729	141,501	276,904
Standard Commercial Lines [Member]
Statement [Line Items]
Underwriting income (loss), before federal income tax	157,306	143,696	198,596
Underwriting income (loss), after federal income tax	$ 124,272	$ 113,520	$ 156,891
Combined ratio	94.90%	94.80%	91.90%
Return on equity contribution	5.00%	4.60%	5.90%
Standard Personal Lines
Statement [Line Items]
Underwriting income (loss), before federal income tax	$ (79,263)	$ (7,193)	$ 3,966
Underwriting income (loss), after federal income tax	$ (62,618)	$ (5,682)	$ 3,133
Combined ratio	121.70%	102.40%	98.60%
Return on equity contribution	(2.50%)	(0.20%)	0.10%
E&S Lines [Member]
Statement [Line Items]
Underwriting income (loss), before federal income tax	$ 54,756	$ 30,299	$ 16,030
Underwriting income (loss), after federal income tax	$ 43,257	$ 23,936	$ 12,664
Combined ratio	86.00%	90.90%	94.30%
Return on equity contribution	1.70%	1.00%	0.50%
Investments SIGI
Statement [Line Items]
Return on equity contribution	12.40%	9.40%	9.90%