Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Total net premiums earned	$ 507,390	$ 462,639	$ 1,473,822	$ 1,382,759
Net investment income earned	32,061	34,292	91,208	106,600
Net realized investment gains	308	15,231	15,771	26,988
Total investment revenues	32,369	49,523	106,979	133,588
Other income	698	3,196	5,521	14,931
Total revenues	540,457	515,358	1,586,322	1,531,278
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	540,457	515,358	1,586,322	1,531,270
Other Income [Member]
Segment Reporting Information [Line Items]
Other income	0	0	0	8
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	389,990	354,973	1,136,960	1,069,406
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	90,758	83,536	265,771	249,224
Standard Commercial Lines [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	74,560	66,732	213,991	205,137
Standard Commercial Lines [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	123,252	110,894	357,430	331,303
Standard Commercial Lines [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	68,587	61,304	199,699	182,716
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	23,726	21,649	69,603	63,797
Standard Commercial Lines [Member] | Bonds Segment [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	5,031	4,791	15,137	14,281
Standard Commercial Lines [Member] | Other Commercial [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	3,628	3,237	10,649	9,633
Standard Commercial Lines [Member] | Miscellaneous Income [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	448	2,830	4,680	13,315
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	72,338	74,743	217,479	225,347
Standard Personal Lines [Member] | Miscellaneous Income [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	250	366	841	1,608
Standard Personal Lines [Member] | Personal Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	36,623	37,695	110,373	113,943
Standard Personal Lines [Member] | Homeowners [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	33,670	33,957	101,122	100,831
Standard Personal Lines [Member] | Other Personal [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	1,795	2,725	5,143	8,965
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	45,760	36,119	124,904	102,929
Excess And Surplus Operations [Member] | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	2,056	1,412	5,562	3,859
Excess And Surplus Operations [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	32,395	24,659	87,914	70,526
Excess And Surplus Operations [Member] | Commercial Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 11,309	$ 10,048	$ 31,428	$ 28,544