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Segment Information (Reconciliation of Segment Results to Income from Continuing Operations, before Federal Income Tax (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
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax	$ 35,168	$ 32,033	$ 43,777	$ 40,955	$ 49,053	$ 44,831	$ 29,124	$ 26,021
Total investment revenues									$ 125,817	$ 134,487	$ 165,307
Interest expense									(22,771)	(22,428)	(23,063)
General corporate and other expenses									(35,024)	(28,396)	(23,256)
Income from continuing operations, before federal income tax									219,955	232,692	197,131
Standard Commercial Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax									146,435	164,496	61,221
Standard Personal Lines [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax									12,419	1,336	16,536
Excess And Surplus Operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Underwriting gain (loss), before federal income tax									(6,921)	(16,803)	386
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments									$ 277,750	$ 283,516	$ 243,450

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
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-Topic 225
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