Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Total net premiums earned	$ 812,283	$ 724,960
Miscellaneous income	1,529	4,112
Net investment income	72,602	69,716
Net realized and unrealized investment (losses) gains	(40,352)	5,119
Total Investments revenue	32,250	74,835
Total revenues	846,062	803,907
Standard Commercial Lines
Segment Reporting Information [Line Items]
Total revenues	662,570	592,848
Standard Commercial Lines | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	120,062	102,810
Standard Commercial Lines | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	84,680	78,190
Standard Commercial Lines | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	216,325	193,520
Standard Commercial Lines | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	193,830	171,881
Standard Commercial Lines | Business Owners Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	30,044	28,627
Standard Commercial Lines | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	10,360	8,593
Standard Commercial Lines | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	6,168	5,520
Standard Commercial Lines | Miscellaneous income
Segment Reporting Information [Line Items]
Miscellaneous income	1,101	3,707
Standard Personal Lines
Segment Reporting Information [Line Items]
Total revenues	73,070	74,226
Standard Personal Lines | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	39,716	41,393
Standard Personal Lines | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	31,187	30,598
Standard Personal Lines | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,739	1,830
Standard Personal Lines | Miscellaneous income
Segment Reporting Information [Line Items]
Miscellaneous income	428	405
E&S Lines
Segment Reporting Information [Line Items]
Total revenues	78,172	61,998
E&S Lines | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	54,624	43,833
E&S Lines | Property Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 23,548	$ 18,165