Segment Information (Income Before and After 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 Information [Line Items]
Net investment income	$ 68,185	$ 55,826	$ 158,596	$ 164,949
Net realized and unrealized investment gains (losses)	7,721	(2,183)	(24,296)	15,295
Total investment segment income, before federal income tax	75,906	53,643	134,300	180,244
Tax on investment income	14,676	10,884	24,338	34,733
Total investment segment income, after federal income tax	61,230	42,759	109,962	145,511
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	42,718	30,016	76,574	92,974
Underwriting gain (loss), after federal income tax	$ 33,746	$ 23,713	$ 60,493	$ 73,450
Combined ratio	92.30%	94.20%	95.10%	93.90%
ROE Contribution	5.70%	4.50%	3.50%	5.00%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ (14,404)	$ (631)	$ (20,342)	$ 7,074
Underwriting gain (loss), after federal income tax	$ (11,379)	$ (499)	$ (16,070)	$ 5,588
Combined ratio	119.00%	100.80%	109.10%	96.90%
ROE Contribution	(1.90%)	(0.10%)	(0.90%)	0.40%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ (7,277)	$ 1,916	$ (3,965)	$ 9,439
Underwriting gain (loss), after federal income tax	$ (5,748)	$ 1,514	$ (3,132)	$ 7,457
Combined ratio	112.00%	96.90%	102.20%	94.70%
ROE Contribution	(1.00%)	0.30%	(0.20%)	0.50%
Investments [Member]
Segment Reporting Information [Line Items]
ROE Contribution	9.40%	8.60%	7.50%	9.10%