Segment Information (Gross Premiums Earned and Reconciliation to Net Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Segment net premiums earned	$ 207,293	$ 209,149	$ 411,149	$ 417,298
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned	(18,451)	(19,158)	(37,114)	(38,107)
Segment net premiums earned	127,096	126,011	247,456	250,079
Specialty P&C | HCPL
Segment Reporting Information [Line Items]
Gross premiums earned	111,836	108,297	215,304	215,761
Specialty P&C | Small Business Unit
Segment Reporting Information [Line Items]
Gross premiums earned	24,915	27,307	51,565	54,425
Specialty P&C | Medical Technology Liability
Segment Reporting Information [Line Items]
Gross premiums earned	8,641	8,356	17,169	16,658
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned	155	1,209	532	1,342
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(21,024)	(24,308)	(42,121)	(48,646)
Segment net premiums earned	42,406	46,574	86,921	92,512
Workers' Compensation Insurance | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned	45,524	50,036	93,008	99,321
Workers' Compensation Insurance | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned	17,906	20,846	36,034	41,837
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(2,164)	(2,414)	(4,374)	(4,851)
Segment net premiums earned	16,748	19,284	33,728	38,787
Segregated Portfolio Cell Reinsurance | HCPL
Segment Reporting Information [Line Items]
Gross premiums earned	1,723	1,349	3,400	2,672
Segregated Portfolio Cell Reinsurance | Other
Segment Reporting Information [Line Items]
Gross premiums earned	0	120	0	240
Segregated Portfolio Cell Reinsurance | Workers' compensation
Segment Reporting Information [Line Items]
Gross premiums earned	17,189	20,229	34,702	40,726
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Gross premiums earned	26,336	21,997	54,532	45,825
Ceded premiums earned	(5,293)	(4,717)	(11,488)	(9,905)
Segment net premiums earned	$ 21,043	$ 17,280	$ 43,044	$ 35,920