Segment Information (Revenue by Segment) (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]
Total net premiums earned	$ 694,541	$ 653,620	$ 1,976,915	$ 1,928,812
Miscellaneous income	6,119	3,162	12,627	8,535
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 Investments revenue	75,906	53,643	134,300	180,244
Total revenues	776,566	710,425	2,123,842	2,117,591
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	5,521	2,782	11,107	7,544
Total revenues	563,606	517,943	1,586,776	1,526,538
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	160,937	141,182	449,162	408,706
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	75,595	75,478	204,207	232,657
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	181,459	169,084	509,312	495,402
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	97,997	89,215	287,279	262,418
Standard Commercial Lines [Member] | Business Owners Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	27,605	26,371	82,157	78,624
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	9,226	8,990	28,075	26,861
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,266	4,841	15,477	14,326
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	598	380	1,520	991
Total revenues	76,574	77,118	225,257	232,149
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	42,458	43,226	123,134	129,777
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	31,395	31,529	94,537	95,672
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	2,123	1,983	6,066	5,709
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Total revenues	60,480	61,721	177,509	178,660
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	43,650	47,171	130,444	136,455
Excess And Surplus Operations [Member] | Property Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 16,830	$ 14,550	$ 47,065	$ 42,205