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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total net premiums earned	$ 642,619	$ 604,836	$ 1,275,192	$ 1,196,664
Miscellaneous income	3,053	3,179	5,373	5,358
Net investment income	58,505	45,553	109,123	88,784
Net realized and unrealized investment gains (losses)	4,027	(1,652)	17,478	(12,201)
Total investment revenues	62,532	43,901	126,601	76,583
Total revenues	708,204	651,916	1,407,166	1,278,605
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	2,718	2,882	4,762	4,708
Total revenues	509,367	478,894	1,008,595	946,084
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	136,338	122,104	267,524	240,335
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	78,464	80,021	157,179	158,844
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	164,793	153,002	326,318	302,831
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	87,136	82,162	173,203	162,488
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	26,172	25,829	52,253	51,420
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	8,967	8,335	17,871	16,469
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	4,779	4,559	9,485	8,989
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	335	297	611	649
Total revenues	77,448	75,974	155,031	150,582
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	43,388	41,810	86,551	82,252
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	32,013	32,223	64,143	64,424
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,712	1,644	3,726	3,257
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	0	0	0	1
Total revenues	58,857	53,147	116,939	105,356
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	44,756	39,379	89,284	77,919
Excess And Surplus Operations [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 14,101	$ 13,768	$ 27,655	$ 27,436

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