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Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue from External Customer [Line Items]
Net premiums earned	$ 516,087	$ 507,390	$ 490,309	$ 476,123	$ 469,850	$ 462,639	$ 463,625	$ 456,495	$ 1,989,909	$ 1,852,609	$ 1,736,072
Other income									7,456	16,945	12,294
Net investment income	$ 30,108	$ 32,061	$ 32,230	$ 26,917	$ 32,108	$ 34,292	$ 36,774	$ 35,534	121,316	138,708	134,643
Net realized gains									13,171	26,599	20,732
Total investment revenues									134,487	165,307	155,375
Total all segments									2,131,852	2,034,514	1,903,648
Total revenues									2,131,852	2,034,861	1,903,741
Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									7,456	16,598	12,201
Other Income [Member]
Revenue from External Customer [Line Items]
Other income									0	347	93
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									358,909	333,310	310,994
Standard Commercial Lines [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Net premiums earned									290,075	274,585	267,612
Standard Commercial Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									483,291	444,938	405,322
Standard Commercial Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									269,022	244,792	224,412
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Revenue from External Customer [Line Items]
Net premiums earned									93,428	85,788	77,097
Standard Commercial Lines [Member] | Bonds Segment [Member]
Revenue from External Customer [Line Items]
Net premiums earned									20,350	19,288	19,000
Standard Commercial Lines [Member] | Other Commercial [Member]
Revenue from External Customer [Line Items]
Net premiums earned									14,367	13,011	12,182
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									6,343	14,747	10,253
Standard Commercial Lines [Member] | Total standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,535,785	1,430,459	1,326,872
Property and Casualty, Personal Insurance Product Line [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Net premiums earned									1,113	1,834	1,948
Property and Casualty, Personal Insurance Product Line [Member] | Personal Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									146,784	151,317	152,005
Property and Casualty, Personal Insurance Product Line [Member] | Homeowners [Member]
Revenue from External Customer [Line Items]
Net premiums earned									134,382	134,273	127,991
Property and Casualty, Personal Insurance Product Line [Member] | Other Personal [Member]
Revenue from External Customer [Line Items]
Net premiums earned									6,968	11,157	14,336
Property and Casualty, Personal Insurance Product Line [Member] | Property and Casualty, Personal Insurance Product Line [Member]
Revenue from External Customer [Line Items]
Net premiums earned									289,247	298,581	296,280
E&S Lines [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Net premiums earned									7,795	5,436	4,306
E&S Lines [Member] | General Liability [Member]
Revenue from External Customer [Line Items]
Net premiums earned									121,802	96,142	88,761
E&S Lines [Member] | Commercial Property [Member]
Revenue from External Customer [Line Items]
Net premiums earned									42,736	38,572	32,054
E&S Lines [Member] | Miscellaneous Income [Member]
Revenue from External Customer [Line Items]
Other income									0	17	0
E&S Lines [Member] | Total standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									$ 172,333	$ 140,167	$ 125,121

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- 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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