Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Total net premiums earned	$ 490,309	$ 463,625	$ 966,432	$ 920,120
Net investment income earned	32,230	36,774	59,147	72,308
Net realized investment gains	(3,420)	4,539	15,463	11,757
Total investment revenues	28,810	41,313	74,610	84,065
Other income	2,854	1,911	4,823	11,735
Total revenues	521,973	506,849	1,045,865	1,015,920
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	521,973	506,846	1,045,865	1,015,912
Other Income [Member]
Segment Reporting Information [Line Items]
Other income	0	3	0	8
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	379,768	356,078	746,970	714,433
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	88,658	83,472	175,013	165,688
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	70,954	68,992	139,431	138,405
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	119,207	111,591	234,178	220,409
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	66,549	61,226	131,112	121,412
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	23,178	21,279	45,877	42,148
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,106	4,734	10,106	9,490
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	3,553	3,213	7,021	6,396
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	2,563	1,571	4,232	10,485
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	72,362	74,881	145,141	150,604
Standard Personal Lines [Member] | Miscellaneous Income [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	291	337	591	1,242
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	36,740	38,022	73,750	76,248
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	33,731	33,576	67,452	66,874
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,600	2,946	3,348	6,240
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	41,033	34,574	79,144	66,810
Excess And Surplus Operations [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,776	1,259	3,506	2,447
Excess And Surplus Operations [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	28,793	23,936	55,519	45,867
Excess And Surplus Operations [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 10,464	$ 9,379	$ 20,119	$ 18,496