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Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Net premiums earned	$ 552,753	$ 542,429	$ 531,932	$ 522,458	$ 516,087	$ 507,390	$ 490,309	$ 476,123	$ 2,149,572	$ 1,989,909	$ 1,852,609
Other income									8,881	7,456	16,945
Net investment income	$ 35,428	$ 33,375	$ 31,182	$ 30,769	$ 30,108	$ 32,061	$ 32,230	$ 26,917	130,754	121,316	138,708
Realized investment (losses) gains									(4,937)	13,171	26,599
Total investment revenues									125,817	134,487	165,307
Total all segments									2,284,270	2,131,852	2,034,514
Total revenues									2,284,270	2,131,852	2,034,861
Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									8,881	7,456	16,598
Other Income [Member]
Revenue from External Customer [Line Items]
Other income									0	0	347
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									398,942	358,909	333,310
Standard Commercial Lines [Member] | Workers Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned									308,233	290,075	274,585
Standard Commercial Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									527,859	483,291	444,938
Standard Commercial Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									293,438	269,022	244,792
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Revenue from External Customer [Line Items]
Net premiums earned									97,754	93,428	85,788
Standard Commercial Lines [Member] | Bonds Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned									23,227	20,350	19,288
Standard Commercial Lines [Member] | Other Commercial [Member]
Revenue from External Customer [Line Items]
Net premiums earned									16,030	14,367	13,011
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									7,782	6,343	14,747
Standard Commercial Lines [Member] | Total standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,673,265	1,535,785	1,430,459
Standard Personal Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,098	1,113	1,834
Standard Personal Lines [Member] | Personal Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									142,876	146,784	151,317
Standard Personal Lines [Member] | Homeowners [Member]
Revenue from External Customer [Line Items]
Net premiums earned									130,973	134,382	134,273
Standard Personal Lines [Member] | Other Personal [Member]
Revenue from External Customer [Line Items]
Net premiums earned									6,758	6,968	11,157
Standard Personal Lines [Member] | Standard Personal Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									281,705	289,247	298,581
E&S Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									151,638	126,064	99,086
E&S Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									51,844	46,269	41,064
E&S Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									1	0	17
E&S Lines [Member] | Total standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									$ 203,483	$ 172,333	$ 140,167

X

- Definition

Amount of total revenue for the reportable segments, including revenue from external customers, transactions with other operating segments of the same entity, and (gross) interest revenue. All considered to be "operating revenue" for purposes of the taxonomy.

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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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