Segment Information (Gross Premiums Earned and Reconciliation to Net Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Segment net premiums earned	$ 203,855	$ 208,149
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned	(18,664)	(18,952)
Segment net premiums earned	120,359	124,067
Specialty P&C | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned	123,366	128,021
Specialty P&C | Legal professional liability
Segment Reporting Information [Line Items]
Gross premiums earned	6,751	6,560
Specialty P&C | Medical technology liability
Segment Reporting Information [Line Items]
Gross premiums earned	8,529	8,302
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned	377	136
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(21,098)	(24,337)
Segment net premiums earned	44,515	45,939
Workers' Compensation Insurance | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned	47,485	49,285
Workers' Compensation Insurance | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned	18,128	20,991
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Ceded premiums earned	(2,210)	(2,437)
Segment net premiums earned	16,980	19,502
Segregated Portfolio Cell Reinsurance | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned	1,677	1,323
Segregated Portfolio Cell Reinsurance | Other
Segment Reporting Information [Line Items]
Gross premiums earned	0	120
Segregated Portfolio Cell Reinsurance | Workers' compensation
Segment Reporting Information [Line Items]
Gross premiums earned	17,513	20,496
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Gross premiums earned	28,196	23,828
Ceded premiums earned	(6,195)	(5,187)
Segment net premiums earned	$ 22,001	$ 18,641