Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from External Customer [Line Items]
Net premiums earned	$ 704,899	$ 694,541	$ 630,671	$ 651,703	$ 668,359	$ 653,620	$ 642,619	$ 632,573	$ 2,681,814	$ 2,597,171	$ 2,436,229
Other income	4,943	6,119	4,683	1,825	3,820	3,162	3,053	2,320	17,570	12,355	9,438
Net investment income	68,511	68,185	34,444	55,967	57,594	55,826	58,505	50,618	227,107	222,543	195,336
Net realized and unrealized investment (losses) gain	20,079	7,721	12,649	(44,666)	(873)	(2,183)	4,027	13,451	(4,217)	14,422	(54,923)
Total investment revenues									222,890	236,965	140,413
Total revenues	$ 798,432	$ 776,566	$ 682,447	$ 664,829	$ 728,900	$ 710,425	$ 708,204	$ 698,962	2,922,274	2,846,491	2,586,080
Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Other income									15,512	10,889	8,180
Total revenues									2,158,696	2,060,503	1,920,402
Standard Personal Lines
Revenue from External Customer [Line Items]
Other income									2,058	1,466	1,257
Total revenues									301,198	309,205	305,698
E&S Lines [Member]
Revenue from External Customer [Line Items]
Other income									0	0	1
Total revenues									239,490	239,818	219,567
Commercial Automobile [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									615,181	554,256	493,093
Workers Compensation [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									278,062	311,370	317,616
General Liability [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									694,019	669,895	616,187
Commercial Property [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									388,120	353,834	329,660
Business Owners' Policies [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									110,210	105,252	103,412
Bonds Segment [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									36,742	35,726	33,991
Other Commercial [Member] | Standard Commercial Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									20,850	19,281	18,263
Personal Automobile [Member] | Standard Personal Lines
Revenue from External Customer [Line Items]
Net premiums earned									165,020	172,606	168,250
Homeowners [Member] | Standard Personal Lines
Revenue from External Customer [Line Items]
Net premiums earned									125,405	127,543	128,961
Other Personal [Member] | Standard Personal Lines
Revenue from External Customer [Line Items]
Net premiums earned									8,715	7,590	7,230
Casualty Insurance Product Line [Member] | E&S Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									174,408	182,864	164,313
Property Insurance [Member] | E&S Lines [Member]
Revenue from External Customer [Line Items]
Net premiums earned									$ 65,082	$ 56,954	$ 55,253