Unpaid Losses and Settlement Expenses (Summary of Cumulative Paid Losses and Settlement Expenses) (Details) $ in Thousands	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	$ 255,841
Cumulative paid loss and settlement expenses, net of reinsurance	207,405
Unpaid losses and settlement expense	44,521
Unpaid loss and settlement expense, net of reinsurance	48,556
Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	98,421
Cumulative paid loss and settlement expenses, net of reinsurance	94,362
Unpaid loss and settlement expense, net of reinsurance	4,035
Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	157,420
Cumulative paid loss and settlement expenses, net of reinsurance	113,043
Unpaid loss and settlement expense, net of reinsurance	44,521
2011 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	18,326	$ 18,341	$ 18,355	$ 18,450	$ 18,672	$ 18,831	$ 18,791	$ 18,515	$ 18,834	$ 19,802
Total IBNR plus expected development on reported claims	$ (10)
Cumulative number of reported claims | claim	2,006
Cumulative paid loss and settlement expenses, net of reinsurance	$ 18,306	18,302	18,250	18,107	17,560	16,678	14,592	12,300	10,112	6,996
2011 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	6,620	6,621	6,632	6,631	6,645	6,733	6,752	6,621	6,708	7,427
Total IBNR plus expected development on reported claims	$ 1
Cumulative number of reported claims | claim	904
Cumulative paid loss and settlement expenses, net of reinsurance	$ 6,620	6,620	6,623	6,623	6,589	6,556	6,520	6,459	6,351	5,327
2011 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	11,706	11,720	11,723	11,819	12,027	12,098	12,039	11,894	12,126	12,375
Total IBNR plus expected development on reported claims	$ (11)
Cumulative number of reported claims | claim	1,102
Cumulative paid loss and settlement expenses, net of reinsurance	$ 11,686	11,682	11,627	11,484	10,971	10,122	8,072	5,841	3,761	$ 1,669
2012 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	18,962	19,003	19,087	19,256	19,174	18,184	17,813	17,712	19,265
Total IBNR plus expected development on reported claims	$ 9
Cumulative number of reported claims | claim	1,823
Cumulative paid loss and settlement expenses, net of reinsurance	$ 18,874	18,748	18,625	18,225	17,239	14,689	11,958	9,422	6,129
2012 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	$ 6,161	6,167	6,455	6,411	6,482	6,546	6,406	6,374	6,143
Cumulative number of reported claims | claim	668
Cumulative paid loss and settlement expenses, net of reinsurance	$ 6,176	6,176	6,469	6,472	6,326	6,362	6,369	6,401	4,949
2012 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	12,801	12,836	12,632	12,845	12,692	11,638	11,407	11,338	13,122
Total IBNR plus expected development on reported claims	$ 9
Cumulative number of reported claims | claim	1,155
Cumulative paid loss and settlement expenses, net of reinsurance	$ 12,698	12,572	12,156	11,753	10,913	8,327	5,589	3,021	$ 1,180
2013 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	22,351	22,348	22,140	22,167	21,375	21,459	21,861	21,850
Total IBNR plus expected development on reported claims	$ 38
Cumulative number of reported claims | claim	1,771
Cumulative paid loss and settlement expenses, net of reinsurance	$ 22,011	21,836	21,252	20,446	18,661	15,834	12,235	8,435
2013 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	8,270	8,272	8,282	8,471	8,415	8,290	8,302	9,266
Total IBNR plus expected development on reported claims	$ 15
Cumulative number of reported claims | claim	626
Cumulative paid loss and settlement expenses, net of reinsurance	$ 8,270	8,271	8,272	8,265	8,238	8,200	8,079	6,856
2013 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	14,081	14,076	13,858	13,696	12,960	13,169	13,559	12,584
Total IBNR plus expected development on reported claims	$ 23
Cumulative number of reported claims | claim	1,145
Cumulative paid loss and settlement expenses, net of reinsurance	$ 13,741	13,565	12,980	12,181	10,423	7,634	4,156	$ 1,579
2014 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	24,737	24,781	24,698	24,601	23,139	22,330	22,250
Total IBNR plus expected development on reported claims	$ 113
Cumulative number of reported claims | claim	1,953
Cumulative paid loss and settlement expenses, net of reinsurance	$ 24,175	23,707	23,027	21,398	17,261	11,718	7,782
2014 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	7,784	7,785	7,817	7,883	7,798	7,586	8,865
Total IBNR plus expected development on reported claims	$ 6
Cumulative number of reported claims | claim	740
Cumulative paid loss and settlement expenses, net of reinsurance	$ 7,801	7,795	7,795	7,796	7,746	7,631	6,243
2014 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	16,953	16,996	16,881	16,718	15,341	14,744	13,385
Total IBNR plus expected development on reported claims	$ 107
Cumulative number of reported claims | claim	1,213
Cumulative paid loss and settlement expenses, net of reinsurance	$ 16,374	15,912	15,232	13,602	9,515	4,087	$ 1,539
2015 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	22,350	22,140	21,496	21,157	21,370	24,289
Total IBNR plus expected development on reported claims	$ 89
Cumulative number of reported claims | claim	1,648
Cumulative paid loss and settlement expenses, net of reinsurance	$ 20,918	20,145	18,172	14,874	11,359	6,462
2015 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	7,636	7,654	7,634	7,717	7,494	7,693
Total IBNR plus expected development on reported claims	$ 21
Cumulative number of reported claims | claim	554
Cumulative paid loss and settlement expenses, net of reinsurance	$ 7,657	7,660	7,645	7,474	7,040	5,057
2015 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	14,714	14,486	13,862	13,440	13,876	16,596
Total IBNR plus expected development on reported claims	$ 68
Cumulative number of reported claims | claim	1,094
Cumulative paid loss and settlement expenses, net of reinsurance	$ 13,261	12,485	10,527	7,400	4,319	$ 1,405
2016 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	25,951	25,236	24,612	22,824	25,618
Total IBNR plus expected development on reported claims	$ 332
Cumulative number of reported claims | claim	1,616
Cumulative paid loss and settlement expenses, net of reinsurance	$ 22,643	20,561	16,425	13,109	7,647
2016 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	8,404	8,381	8,372	7,981	8,941
Total IBNR plus expected development on reported claims	$ 41
Cumulative number of reported claims | claim	576
Cumulative paid loss and settlement expenses, net of reinsurance	$ 8,437	8,356	8,236	7,624	6,157
2016 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	17,547	16,855	16,240	14,843	16,677
Total IBNR plus expected development on reported claims	$ 291
Cumulative number of reported claims | claim	1,040
Cumulative paid loss and settlement expenses, net of reinsurance	$ 14,206	12,205	8,189	5,485	$ 1,490
2017 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	29,802	29,583	29,371	29,801
Total IBNR plus expected development on reported claims	$ 760
Cumulative number of reported claims | claim	1,753
Cumulative paid loss and settlement expenses, net of reinsurance	$ 25,473	22,363	18,901	11,578
2017 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	13,825	13,741	13,568	13,993
Total IBNR plus expected development on reported claims	$ 71
Cumulative number of reported claims | claim	715
Cumulative paid loss and settlement expenses, net of reinsurance	$ 13,595	13,610	13,482	10,055
2017 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	15,977	15,842	15,803	15,808
Total IBNR plus expected development on reported claims	$ 689
Cumulative number of reported claims | claim	1,038
Cumulative paid loss and settlement expenses, net of reinsurance	$ 11,878	8,753	5,419	$ 1,523
2018 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	28,048	28,236	29,762
Total IBNR plus expected development on reported claims	$ 2,888
Cumulative number of reported claims | claim	1,842
Cumulative paid loss and settlement expenses, net of reinsurance	$ 20,337	16,665	10,451
2018 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	10,966	11,114	11,454
Total IBNR plus expected development on reported claims	$ 59
Cumulative number of reported claims | claim	725
Cumulative paid loss and settlement expenses, net of reinsurance	$ 11,025	11,009	8,487
2018 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	17,082	17,122	18,308
Total IBNR plus expected development on reported claims	$ 2,829
Cumulative number of reported claims | claim	1,117
Cumulative paid loss and settlement expenses, net of reinsurance	$ 9,312	5,656	$ 1,964
2019 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	33,958	33,564
Total IBNR plus expected development on reported claims	$ 6,041
Cumulative number of reported claims | claim	1,862
Cumulative paid loss and settlement expenses, net of reinsurance	$ 21,614	15,285
2019 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	14,758	13,933
Total IBNR plus expected development on reported claims	$ (180)
Cumulative number of reported claims | claim	828
Cumulative paid loss and settlement expenses, net of reinsurance	$ 14,161	11,621
2019 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	19,200	19,631
Total IBNR plus expected development on reported claims	$ 6,221
Cumulative number of reported claims | claim	1,034
Cumulative paid loss and settlement expenses, net of reinsurance	$ 7,453	$ 3,664
2020 [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	31,356
Total IBNR plus expected development on reported claims	$ 10,383
Cumulative number of reported claims | claim	1,411
Cumulative paid loss and settlement expenses, net of reinsurance	$ 13,054
2020 [Member] | Property Line [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	13,997
Total IBNR plus expected development on reported claims	$ (20)
Cumulative number of reported claims | claim	885
Cumulative paid loss and settlement expenses, net of reinsurance	$ 10,620
2020 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Incurred loss and settlement expenses, net of reinsurance	17,359
Total IBNR plus expected development on reported claims	$ 10,403
Cumulative number of reported claims | claim	526
Cumulative paid loss and settlement expenses, net of reinsurance	$ 2,434
Years 2011 Through 2020 [Member]
Claims Development [Line Items]
Unpaid losses and settlement expense	48,436
Years 2011 Through 2020 [Member] | Property Line [Member]
Claims Development [Line Items]
Unpaid losses and settlement expense	4,059
Years 2011 Through 2020 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Unpaid losses and settlement expense	44,377
Prior To 2011 [Member]
Claims Development [Line Items]
Unpaid losses and settlement expense	120
Prior To 2011 [Member] | Property Line [Member]
Claims Development [Line Items]
Unpaid losses and settlement expense	(24)
Prior To 2011 [Member] | Workers Compensation and Liability Lines [Member]
Claims Development [Line Items]
Unpaid losses and settlement expense	$ 144