Segment Information (Gross Premiums Earned and Reconciliation to Net Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Segment net premiums earned	$ 194,559	$ 215,788	$ 605,708	$ 633,086
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned	(19,195)	(21,277)	(56,309)	(59,384)
Segment net premiums earned	117,849	125,237	365,305	375,315
Specialty P&C | HCPL
Segment Reporting Information [Line Items]
Gross premiums earned	101,785	109,484	317,088	325,245
Specialty P&C | Small Business Unit
Segment Reporting Information [Line Items]
Gross premiums earned	26,372	27,920	77,937	82,341
Specialty P&C | Medical Technology Liability
Segment Reporting Information [Line Items]
Gross premiums earned	8,749	8,519	25,918	25,177
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned	138	591	671	1,936
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(20,291)	(24,926)	(62,412)	(73,571)
Segment net premiums earned	42,516	49,477	129,437	141,990
Workers' Compensation Insurance | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned	45,620	53,156	138,628	152,477
Workers' Compensation Insurance | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned	17,187	21,247	53,221	63,084
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(2,123)	(2,439)	(6,497)	(7,290)
Segment net premiums earned	16,052	19,779	49,780	58,566
Segregated Portfolio Cell Reinsurance | HCPL
Segment Reporting Information [Line Items]
Gross premiums earned	1,699	1,399	5,099	4,071
Segregated Portfolio Cell Reinsurance | Other
Segment Reporting Information [Line Items]
Gross premiums earned	0	240	0	480
Segregated Portfolio Cell Reinsurance | Workers' compensation
Segment Reporting Information [Line Items]
Gross premiums earned	16,476	20,579	51,178	61,305
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Gross premiums earned	22,777	26,399	77,309	72,225
Ceded premiums earned	(4,635)	(5,104)	(16,123)	(15,010)
Segment net premiums earned	$ 18,142	$ 21,295	$ 61,186	$ 57,215