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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total net premiums earned	$ 632,573	$ 591,828
Other income	2,320	2,179
Net investment income	50,618	43,231
Net realized and unrealized investment gains (losses)	13,451	(10,549)
Total investment revenues	64,069	32,682
Total revenues	698,962	626,689
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Other income	2,044	1,826
Total revenues	499,228	467,190
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	131,186	118,231
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	78,715	78,823
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	161,525	149,829
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	86,067	80,326
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	26,081	25,591
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	8,904	8,134
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	4,706	4,430
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Other income	276	352
Total revenues	77,583	74,608
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	43,163	40,442
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	32,130	32,201
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	2,014	1,613
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Other income	0	1
Total revenues	58,082	52,209
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	44,528	38,540
Excess And Surplus Operations [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 13,554	$ 13,668

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