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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total net premiums earned	$ 630,671	$ 642,619	$ 1,282,374	$ 1,275,192
Miscellaneous income	4,683	3,053	6,508	5,373
Net investment income	34,444	58,505	90,411	109,123
Net realized and unrealized investment gains (losses)	12,649	4,027	(32,017)	17,478
Total Investments revenue	47,093	62,532	58,394	126,601
Total revenues	682,447	708,204	1,347,276	1,407,166
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	4,195	2,718	5,586	4,762
Total revenues	505,199	509,367	1,023,170	1,008,595
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	138,535	136,338	288,225	267,524
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	61,906	78,464	128,612	157,179
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	163,273	164,793	327,853	326,318
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	95,413	87,136	189,282	173,203
Standard Commercial Lines [Member] | Business Owners Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	27,516	26,172	54,552	52,253
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	9,210	8,967	18,849	17,871
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,151	4,779	10,211	9,485
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	488	335	922	611
Total revenues	72,121	77,448	148,683	155,031
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	38,189	43,388	80,676	86,551
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	31,652	32,013	63,142	64,143
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,792	1,712	3,943	3,726
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	0	0	0	0
Total revenues	58,034	58,857	117,029	116,939
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	42,722	44,756	86,794	89,284
Excess And Surplus Operations [Member] | Property Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 15,312	$ 14,101	$ 30,235	$ 27,655

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