Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total net premiums earned	$ 653,620	$ 614,277	$ 1,928,812	$ 1,810,941
Miscellaneous income	3,162	2,538	8,535	7,896
Net investment income	55,826	52,443	164,949	141,227
Net realized and unrealized investment (losses) gains	(2,183)	(4,787)	15,295	(16,988)
Total investment revenues	53,643	47,656	180,244	124,239
Total revenues	710,425	664,471	2,117,591	1,943,076
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	2,782	2,228	7,544	6,936
Total revenues	517,943	483,284	1,526,538	1,429,368
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	141,182	124,862	408,706	365,197
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	75,478	78,784	232,657	237,628
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	169,084	154,974	495,402	457,805
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	89,215	83,056	262,418	245,544
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	26,371	25,994	78,624	77,414
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	8,990	8,778	26,861	25,247
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	4,841	4,608	14,326	13,597
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	380	310	991	959
Total revenues	77,118	77,467	232,149	228,049
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	43,226	42,772	129,777	125,024
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	31,529	32,293	95,672	96,717
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,983	2,092	5,709	5,349
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	0	0	0	1
Total revenues	61,721	56,064	178,660	161,420
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	47,171	42,179	136,455	120,098
Excess And Surplus Operations [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 14,550	$ 13,885	$ 42,205	$ 41,321