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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Total net premiums earned	$ 651,703	$ 632,573
Miscellaneous income	1,825	2,320
Net investment income	55,967	50,618
Net realized and unrealized investment (losses) gains	(44,666)	13,451
Total Investments revenue	11,301	64,069
Total revenues	664,829	698,962
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	1,391	2,044
Total revenues	517,971	499,228
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	149,690	131,186
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	66,706	78,715
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	164,580	161,525
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	93,869	86,067
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	27,036	26,081
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	9,639	8,904
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,060	4,706
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	434	276
Total revenues	76,562	77,583
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	42,487	43,163
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	31,490	32,130
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	2,151	2,014
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Miscellaneous income	0	0
Total revenues	58,995	58,082
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	44,072	44,528
Excess And Surplus Operations [Member] | Property Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 14,923	$ 13,554

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