Loss and Loss Expense Reserves, Incurred Claims Development (Details) $ in Thousands	Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	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 ($)	Dec. 31, 2007 USD ($)
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Liabilities for claims and claims adjustment expenses, net of reinsurance	$ 308,278
Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	254,157
Total of incurred-but-not-reported liabilities plus expected development on reported claims	48,156
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	164,914
Outstanding liabilities prior to 2007 net of reinsurance	12,640
Liabilities for claims and claims adjustment expenses, net of reinsurance	101,883
Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	414,111
Total of incurred-but-not-reported liabilities plus expected development on reported claims	36,770
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	285,593
Outstanding liabilities prior to 2007 net of reinsurance	4,621
Liabilities for claims and claims adjustment expenses, net of reinsurance	133,139
Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	78,210
Total of incurred-but-not-reported liabilities plus expected development on reported claims	23,502
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	43,510
Outstanding liabilities prior to 2007 net of reinsurance	0
Liabilities for claims and claims adjustment expenses, net of reinsurance	34,700
Accident Year 2007 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	12,399	$ 12,340	$ 12,637	$ 12,175	$ 11,790	$ 11,239	$ 12,023	$ 13,298	$ 11,930	$ 9,050
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 173
Cumulative number of reported claims | Claim	2,828
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 10,401	10,138	9,974	9,571	9,226	8,352	7,805	8,064	6,072	1,525
Accident Year 2007 [Member] | Commercial Liability [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	39,436	39,598	40,516	40,505	40,429	41,216	41,146	41,692	41,803	38,486
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 79
Cumulative number of reported claims | Claim	1,380
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 39,193	39,216	39,160	39,069	39,006	38,621	36,150	32,571	17,302	$ 5,493
Accident Year 2008 [Member] | Workers' Compensation [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	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
Cumulative number of reported claims | Claim	3,649
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	23,482	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	$ 23
Cumulative number of reported claims | Claim	948
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	20,817	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	$ 1,060
Cumulative number of reported claims | Claim	3,784
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	25,320	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	$ 70
Cumulative number of reported claims | Claim	899
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	19,195	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	$ 1,107
Cumulative number of reported claims | Claim	4,222
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	19,377	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	$ 40
Cumulative number of reported claims | Claim	2,398
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 18,866	18,517	18,627	16,052	11,635	7,166	1,649
Accident Year 2010 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	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	270
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	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	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	$ 2,784
Cumulative number of reported claims | Claim	4,543
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	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	$ 89
Cumulative number of reported claims | Claim	2,899
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 34,009	33,255	30,795	23,615	11,350	1,809
Accident Year 2011 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	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	628
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	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	25,616	24,485	24,887	25,544	23,965
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 3,136
Cumulative number of reported claims | Claim	4,477
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	48,708	51,367	49,743	54,269	49,743
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 279
Cumulative number of reported claims | Claim	3,124
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 47,601	45,303	32,942	23,252	3,086
Accident Year 2012 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	14,605	10,157	5,569	9,276	10,041
Total of incurred-but-not-reported liabilities plus expected development on reported claims	2,653
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	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	32,121	29,913	30,638	27,619
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 5,170
Cumulative number of reported claims | Claim	5,255
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	36,371	37,701	39,143	53,817
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 671
Cumulative number of reported claims | Claim	3,739
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 34,481	25,270	15,772	5,167
Accident Year 2013 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	17,694	11,618	13,034	14,370
Total of incurred-but-not-reported liabilities plus expected development on reported claims	3,449
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	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	34,009	36,968	36,768
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 6,908
Cumulative number of reported claims | Claim	5,371
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	52,483	52,254	49,971
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,239
Cumulative number of reported claims | Claim	3,271
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 28,393	9,046	4,023
Accident Year 2014 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	7,259	8,825	12,675
Total of incurred-but-not-reported liabilities plus expected development on reported claims	3,178
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	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	23,115	26,277
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 6,462
Cumulative number of reported claims | Claim	6,219
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	70,174	61,420
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 5,800
Cumulative number of reported claims | Claim	3,086
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 27,582	10,923
Accident Year 2015 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	7,859	11,638
Total of incurred-but-not-reported liabilities plus expected development on reported claims	6,956
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	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	35,240
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 20,695
Cumulative number of reported claims | Claim	5,520
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 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	61,638
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 27,480
Cumulative number of reported claims | Claim	3,193
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 6,843
Accident Year 2016 [Member] | Professional Liability Assumed [Member]
Incurred Claims Development, Net of Reinsurance [Abstract]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	6,368
Total of incurred-but-not-reported liabilities plus expected development on reported claims	6,368
Cumulative Paid Claims Development, Net of Reinsurance [Abstract]
Cumulative paid claims and allocated claim adjustment expense, net of reinsurance	$ 0