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Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from External Customer [Line Items]
Net premiums earned	$ 668,359	$ 653,620	$ 642,619	$ 632,573	$ 625,288	$ 614,277	$ 604,836	$ 591,828	$ 2,597,171	$ 2,436,229	$ 2,291,027
Other income	3,820	3,162	3,053	2,320	1,542	2,538	3,179	2,179	12,355	9,438	10,716
Net investment income	57,594	55,826	58,505	50,618	54,109	52,443	45,553	43,231	222,543	195,336	161,882
Total net realized and unrealized gains (losses)	(873)	(2,183)	4,027	13,451	(37,935)	(4,787)	(1,652)	(10,549)	14,422	(54,923)	6,359
Total investment revenues									236,965	140,413	168,241
Total revenues	$ 728,900	$ 710,425	$ 708,204	$ 698,962	$ 643,004	$ 664,471	$ 651,916	$ 626,689	2,846,491	2,586,080	2,469,984
Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Other income									10,889	8,180	9,488
Total revenues									2,060,503	1,920,402	1,797,987
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									554,256	493,093	442,818
Standard Commercial Lines [Member] | Workers Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned									311,370	317,616	317,982
Standard Commercial Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									669,895	616,187	569,217
Standard Commercial Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									353,834	329,660	311,932
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Revenue from External Customer [Line Items]
Net premiums earned									105,252	103,412	100,266
Standard Commercial Lines [Member] | Bonds Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned									35,726	33,991	29,086
Standard Commercial Lines [Member] | Other Commercial [Member]
Revenue from External Customer [Line Items]
Net premiums earned									19,281	18,263	17,198
Standard Personal Lines [Member]
Revenue from External Customer [Line Items]
Other income									1,466	1,257	1,228
Total revenues									309,205	305,698	290,929
Standard Personal Lines [Member] | Personal Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									172,606	168,250	153,147
Standard Personal Lines [Member] | Homeowners [Member]
Revenue from External Customer [Line Items]
Net premiums earned									127,543	128,961	129,699
Standard Personal Lines [Member] | Other Personal [Member]
Revenue from External Customer [Line Items]
Net premiums earned									7,590	7,230	6,855
E&S Lines [Member]
Revenue from External Customer [Line Items]
Other income									0	1	0
Total revenues									239,818	219,567	212,827
E&S Lines [Member] | Casualty Insurance Product Line [Member]
Revenue from External Customer [Line Items]
Net premiums earned									182,864	164,313	157,366
E&S Lines [Member] | Property Insurance [Member]
Revenue from External Customer [Line Items]
Net premiums earned									$ 56,954	$ 55,253	$ 55,461

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