Reserve for Losses and Loss Adjustment Expenses - Cumulative Paid Losses and Loss Adjustment Expenses (Details) $ in Thousands	Dec. 31, 2019 USD ($) Claim claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Casualty Reinsurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 863,436
All outstanding losses and loss adjustment expenses prior to 2010, net of reinsurance	1,798
Total outstanding losses and loss adjustment expenses, net of reinsurance	283,676
Casualty Reinsurance | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	59,634	$ 58,707	$ 57,013	$ 54,659	$ 52,761	$ 49,263	$ 44,921	$ 38,430	$ 31,500	$ 21,918
Casualty Reinsurance | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	99,511	97,715	94,627	90,287	87,267	78,164	68,616	61,922	48,688
Casualty Reinsurance | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	137,430	133,606	128,567	121,026	113,943	97,215	81,859	73,124
Casualty Reinsurance | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	134,644	127,732	119,256	108,396	93,902	75,094	59,756
Casualty Reinsurance | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	110,374	101,366	89,899	76,302	58,601	41,421
Casualty Reinsurance | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	90,661	80,208	68,002	53,986	40,021
Casualty Reinsurance | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	78,145	65,409	50,905	36,268
Casualty Reinsurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	90,117	72,891	47,739
Casualty Reinsurance | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	50,274	30,903
Casualty Reinsurance | Accident year 2019
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	12,646
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	625,534
All outstanding losses and loss adjustment expenses prior to 2010, net of reinsurance	8,978
Total outstanding losses and loss adjustment expenses, net of reinsurance	$ 568,814
Claims outstanding | claim	60
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 65,915	65,246	64,214	61,307	57,371	52,071	45,641	35,812	26,418	13,673
Claims outstanding | Claim	1,429
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 101,728	99,798	98,684	94,998	90,884	81,696	72,732	53,109	27,684
Claims outstanding | Claim	1,480
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 94,588	88,812	82,545	74,869	63,216	49,604	33,757	6,944
Claims outstanding | Claim	1,806
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 71,035	66,553	59,641	44,421	30,382	14,509	3,867
Claims outstanding | Claim	2,396
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 71,549	58,774	43,891	28,212	16,969	3,412
Claims outstanding | Claim	2,162
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 73,455	55,911	34,801	17,164	4,048
Claims outstanding | Claim	2,531
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 70,105	46,045	22,852	5,180
Claims outstanding | Claim	2,933
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 42,764	22,956	5,290
Claims outstanding | Claim	2,898
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 26,160	6,000
Claims outstanding | Claim	3,935
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2019
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 8,235
Claims outstanding | Claim	4,038
Fronting and Programs | Specialty Admitted Insurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 43,424
All outstanding losses and loss adjustment expenses prior to 2010, net of reinsurance	48,327
Outstanding losses and loss adjustment expenses assumed from involuntary workers compensation pools	400
Total outstanding losses and loss adjustment expenses, net of reinsurance	48,727
Fronting and Programs | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 52	52	52	52	52	52	28
Claims outstanding | Claim	22
Fronting and Programs | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 2,916	2,854	2,728	2,369	1,687	883
Claims outstanding | Claim	857
Fronting and Programs | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 7,329	6,919	6,165	4,666	2,058
Claims outstanding | Claim	1,362
Fronting and Programs | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 10,732	6,888	5,123	1,894
Claims outstanding | Claim	2,807
Fronting and Programs | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 13,065	6,682	1,223
Claims outstanding | Claim	6,706
Fronting and Programs | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 4,972	885
Claims outstanding | Claim	7,158
Fronting and Programs | Specialty Admitted Insurance | Accident year 2019
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 4,358
Claims outstanding | Claim	5,695
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 167,741
All outstanding losses and loss adjustment expenses prior to 2010, net of reinsurance	1,039
Outstanding losses and loss adjustment expenses assumed from involuntary workers compensation pools	4,656
Total outstanding losses and loss adjustment expenses, net of reinsurance	$ 43,191
Claims outstanding | claim	4
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2010
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 26,365	26,358	26,352	26,345	25,477	25,210	23,898	21,805	16,245	$ 7,157
Claims outstanding | Claim	1,604
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2011
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 34,638	34,614	34,334	34,287	34,240	33,204	29,021	23,127	$ 10,123
Claims outstanding | Claim	1,814
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2012
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 26,994	26,963	26,932	26,897	26,435	24,755	20,308	$ 9,222
Claims outstanding | Claim	1,323
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 10,337	10,309	10,263	10,246	9,846	8,723	4,487
Claims outstanding | Claim	540
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 12,282	12,282	12,236	12,041	10,648	4,633
Claims outstanding | Claim	850
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 15,901	15,889	15,118	13,285	6,604
Claims outstanding | Claim	975
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 12,432	12,135	10,227	4,664
Claims outstanding | Claim	836
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 14,285	12,782	6,546
Claims outstanding | Claim	1,092
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 9,034	4,497
Claims outstanding | Claim	1,230
Individual Risk Workers' Compensation | Specialty Admitted Insurance | Accident year 2019
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 5,473
Claims outstanding | Claim	1,482
Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 535,232
Total outstanding losses and loss adjustment expenses, net of reinsurance	433,053
Commercial Auto | Excess and Surplus Lines | Accident year 2013
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 1,275	1,275	1,275	1,291	1,285	1,182	$ 60
Claims outstanding | Claim	54
Commercial Auto | Excess and Surplus Lines | Accident year 2014
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 17,745	18,678	16,359	12,679	8,645	$ 6,166
Claims outstanding | Claim	7,763
Commercial Auto | Excess and Surplus Lines | Accident year 2015
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 34,819	31,592	24,282	15,234	$ 8,356
Claims outstanding | Claim	41,757
Commercial Auto | Excess and Surplus Lines | Accident year 2016
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 125,108	89,381	54,054	$ 18,295
Claims outstanding | Claim	89,003
Commercial Auto | Excess and Surplus Lines | Accident year 2017
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 192,961	107,377	$ 41,467
Claims outstanding | Claim	133,654
Commercial Auto | Excess and Surplus Lines | Accident year 2018
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 119,099	$ 45,136
Claims outstanding | Claim	95,889
Commercial Auto | Excess and Surplus Lines | Accident year 2019
Claims Development [Line Items]
Cumulative paid losses and loss adjustment expenses, net of reinsurance	$ 44,225
Claims outstanding | Claim	67,380