Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details)) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Investment income	$ 75,906	$ 53,643	$ 134,300	$ 180,244
Interest expense	(7,781)	(7,397)	(23,310)	(26,289)
Corporate expenses	(3,905)	(6,369)	(19,310)	(28,345)
Income before federal income tax	85,257	71,178	143,947	235,097
Standard Commercial Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	42,718	30,016	76,574	92,974
Standard Personal Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	(14,404)	(631)	(20,342)	7,074
Excess And Surplus Operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	(7,277)	1,916	(3,965)	9,439
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total income, before federal income tax all segments	$ 96,943	$ 84,944	$ 186,567	$ 289,731