Segment Information (Gross Premiums Earned and Reconciliation to Net Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Segment net premiums earned	$ 238,993	$ 207,293	$ 426,351	$ 411,149
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned	(17,839)	(18,451)	(34,285)	(37,114)
Segment net premiums earned	168,635	127,096	284,249	247,456
Specialty P&C | HCPL
Segment Reporting Information [Line Items]
Gross premiums earned	150,784	111,836	247,829	215,304
Specialty P&C | Small Business Unit
Segment Reporting Information [Line Items]
Gross premiums earned	25,905	24,915	51,831	51,565
Specialty P&C | Medical Technology Liability
Segment Reporting Information [Line Items]
Gross premiums earned	9,613	8,641	18,551	17,169
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned	172	155	323	532
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(20,084)	(21,024)	(38,706)	(42,121)
Segment net premiums earned	40,626	42,406	80,636	86,921
Workers' Compensation Insurance | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned	43,693	45,524	85,436	93,008
Workers' Compensation Insurance | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned	17,017	17,906	33,906	36,034
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(2,155)	(2,164)	(4,238)	(4,374)
Segment net premiums earned	16,272	16,748	32,156	33,728
Segregated Portfolio Cell Reinsurance | HCPL
Segment Reporting Information [Line Items]
Gross premiums earned	2,173	1,723	4,026	3,400
Segregated Portfolio Cell Reinsurance | Workers' compensation
Segment Reporting Information [Line Items]
Gross premiums earned	16,254	17,189	32,368	34,702
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Gross premiums earned	16,635	26,336	37,020	54,532
Ceded premiums earned	(3,175)	(5,293)	(7,710)	(11,488)
Segment net premiums earned	$ 13,460	$ 21,043	$ 29,310	$ 43,044