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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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total investment revenues									$ 140,413	$ 168,241	$ 125,817
Interest expense									(24,419)	(24,354)	(22,771)
Corporate expenses									(25,446)	(36,255)	(35,024)
Income before federal income tax	$ 52,135	$ 67,130	$ 72,525	$ 19,931	$ 68,150	$ 67,315	$ 58,929	$ 67,574	211,721	261,968	219,955
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									109,104	149,514	146,435
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									12,764	11,104	12,419
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									(695)	(6,282)	(6,921)
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total all segments									$ 261,586	$ 322,577	$ 277,750

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