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Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Net premiums earned	$ 590,088	$ 572,055	$ 568,030	$ 560,854	$ 552,753	$ 542,429	$ 531,932	$ 522,458	$ 2,291,027	$ 2,149,572	$ 1,989,909
Other income	2,190	1,994	3,291	3,241	1,863	2,199	3,868	951	10,716	8,881	7,456
Net investment income	42,587	40,446	41,430	37,419	35,428	33,375	31,182	30,769	161,882	130,754	121,316
Realized investment gains (losses)									6,359	(4,937)	13,171
Total investment revenues									168,241	125,817	134,487
Total revenues	$ 633,737	$ 621,293	$ 614,485	$ 600,469	$ 582,358	$ 581,691	$ 568,747	$ 551,474	2,469,984	2,284,270	2,131,852
Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									10,716	8,881	7,456
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									442,818	398,942	358,909
Standard Commercial Lines [Member] | Workers Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned									317,982	308,233	290,075
Standard Commercial Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									569,217	527,859	483,291
Standard Commercial Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									311,932	293,438	269,022
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Revenue from External Customer [Line Items]
Net premiums earned									100,266	97,754	93,428
Standard Commercial Lines [Member] | Bonds Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned									29,086	23,227	20,350
Standard Commercial Lines [Member] | Other Commercial [Member]
Revenue from External Customer [Line Items]
Net premiums earned									17,198	16,030	14,367
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									9,488	7,782	6,343
Standard Commercial Lines [Member] | Total Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,797,987	1,673,265	1,535,785
Standard Personal Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,228	1,098	1,113
Standard Personal Lines [Member] | Personal Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									153,147	142,876	146,784
Standard Personal Lines [Member] | Homeowners [Member]
Revenue from External Customer [Line Items]
Net premiums earned									129,699	130,973	134,382
Standard Personal Lines [Member] | Other Personal [Member]
Revenue from External Customer [Line Items]
Net premiums earned									6,855	6,758	6,968
Standard Personal Lines [Member] | Standard Personal Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									290,929	281,705	289,247
E&S Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									157,366	151,638	126,064
E&S Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									55,461	51,844	46,269
E&S Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									0	1	0
E&S Lines [Member] | Total E&S Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									$ 212,827	$ 203,483	$ 172,333

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums earned.

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