Loss and Loss Expense Reserves, Incurred Claims Development (Details) $ in Thousands	Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Liabilities for claims and claims adjustment expenses, net of reinsurance	$ 354,282	$ 308,278
Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	283,214
Total of incurred-but-not-reported liabilities plus expected development on reported claims	61,771
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	182,103
Outstanding liabilities prior to 2008 net of reinsurance	12,640
Liabilities for claims and claims adjustment expenses, net of reinsurance	113,751	101,883
Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	491,927
Total of incurred-but-not-reported liabilities plus expected development on reported claims	56,089
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	333,967
Outstanding liabilities prior to 2008 net of reinsurance	4,621
Liabilities for claims and claims adjustment expenses, net of reinsurance	162,581	133,139
Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	83,914
Total of incurred-but-not-reported liabilities plus expected development on reported claims	16,351
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	54,934
Outstanding liabilities prior to 2008 net of reinsurance	0
Liabilities for claims and claims adjustment expenses, net of reinsurance	28,980	34,700
Accident Year 2008 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	25,077	24,352	$ 24,048	$ 23,916	$ 25,109	$ 24,855	$ 24,721	$ 24,715	$ 23,640	$ 19,833
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 661
Number of reported claims per year | Claim	3,656
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 22,910	22,199	21,883	21,306	20,839	20,055	18,943	17,320	13,919	6,046
Accident Year 2008 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	23,733	23,482	23,480	24,902	24,892	23,989	25,511	27,311	28,591	34,693
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 110
Number of reported claims per year | Claim	1,280
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 23,510	23,500	23,444	23,354	23,169	21,396	21,220	15,733	8,942	$ 2,612
Accident Year 2009 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	20,946	20,817	20,641	20,767	21,268	21,261	21,447	20,931	17,270
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 877
Number of reported claims per year | Claim	3,784
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 18,982	18,514	18,028	17,448	16,592	15,576	13,343	10,073	4,186
Accident Year 2009 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	25,485	25,320	25,373	25,872	27,259	26,280	30,203	30,406	29,707
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 136
Number of reported claims per year | Claim	1,161
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 25,199	25,125	25,114	25,066	24,866	23,521	19,718	17,880	$ 928
Accident Year 2010 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	19,541	19,195	18,344	18,849	19,300	19,400	20,111	20,644
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,096
Number of reported claims per year | Claim	4,222
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 16,789	16,590	15,835	14,966	13,845	11,963	9,134	3,974
Accident Year 2010 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	19,081	19,377	19,764	20,300	19,139	21,899	22,161	31,124
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 96
Number of reported claims per year | Claim	2,384
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 18,662	18,866	18,517	18,627	16,052	11,635	7,166	1,649
Accident Year 2010 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	11,893	11,689	10,740	9,733	7,946	7,827	4,277	2,196
Total of incurred-but-not-reported liabilities plus expected development on reported claims	175
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	11,334	11,132	9,904	7,758	5,844	3,505	729	$ 41
Accident Year 2011 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	26,617	27,293	25,515	26,767	26,958	26,628	26,057
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,399
Number of reported claims per year | Claim	4,545
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 22,569	21,622	20,617	18,884	15,973	11,912	4,916
Accident Year 2011 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	36,076	37,122	35,805	36,894	31,633	43,832	46,829
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 271
Number of reported claims per year | Claim	2,764
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 35,561	34,009	33,255	30,795	23,615	11,350	1,809
Accident Year 2011 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	12,744	12,735	10,779	9,859	9,017	8,314	10,492
Total of incurred-but-not-reported liabilities plus expected development on reported claims	290
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	11,679	10,404	8,104	4,983	2,061	637	$ 50
Accident Year 2012 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	27,020	25,616	24,485	24,887	25,544	23,965
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 3,151
Number of reported claims per year | Claim	4,479
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 20,276	18,946	17,415	14,834	11,004	4,597
Accident Year 2012 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	51,475	48,708	51,367	49,743	54,269	49,743
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 478
Number of reported claims per year | Claim	3,211
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 50,036	47,601	45,303	32,942	23,252	3,086
Accident Year 2012 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	16,555	14,605	10,157	5,569	9,276	10,041
Total of incurred-but-not-reported liabilities plus expected development on reported claims	3,049
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	11,239	8,355	5,077	2,388	992	$ 103
Accident Year 2013 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	32,553	32,121	29,913	30,638	27,619
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 5,344
Number of reported claims per year | Claim	5,271
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 23,643	21,655	18,065	12,792	4,880
Accident Year 2013 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	46,690	36,371	37,701	39,143	53,817
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 664
Number of reported claims per year | Claim	3,611
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 44,865	34,481	25,270	15,772	5,167
Accident Year 2013 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	23,256	17,694	11,618	13,034	14,370
Total of incurred-but-not-reported liabilities plus expected development on reported claims	4,653
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	14,779	10,988	5,088	1,135	$ 123
Accident Year 2014 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	33,427	34,009	36,968	36,768
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 5,848
Number of reported claims per year | Claim	5,394
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 22,387	19,075	13,665	5,328
Accident Year 2014 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	52,964	52,483	52,254	49,971
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,451
Number of reported claims per year | Claim	3,091
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 45,075	28,393	9,046	4,023
Accident Year 2014 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	9,837	7,259	8,825	12,675
Total of incurred-but-not-reported liabilities plus expected development on reported claims	2,621
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	3,999	2,241	761	$ 723
Accident Year 2015 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	25,889	23,115	26,277
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 6,921
Number of reported claims per year | Claim	6,284
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 15,020	10,128	2,918
Accident Year 2015 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	64,323	70,174	61,420
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,278
Number of reported claims per year | Claim	3,340
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 49,267	27,582	10,923
Accident Year 2015 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	7,147	7,859	11,638
Total of incurred-but-not-reported liabilities plus expected development on reported claims	3,290
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	1,899	390	$ 10
Accident Year 2016 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	29,757	35,240
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 9,795
Number of reported claims per year | Claim	6,003
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 13,377	5,784
Accident Year 2016 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	68,974	61,638
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 6,725
Number of reported claims per year | Claim	3,581
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 30,377	6,843
Accident Year 2016 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	2,482	6,368
Total of incurred-but-not-reported liabilities plus expected development on reported claims	2,273
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	5	$ 0
Accident Year 2017 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	42,387
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 25,679
Number of reported claims per year | Claim	14,490
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 6,150
Accident Year 2017 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	103,126
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 43,880
Number of reported claims per year | Claim	4,387
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 11,415
Accident Year 2017 [Member] | Professional Liability Reinsurance Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	0
Total of incurred-but-not-reported liabilities plus expected development on reported claims	0
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 0