Segment Information - Gross Premiums Earned by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Ceded premiums earned									$ (124,171)	$ (126,036)	$ (110,347)
Net premiums earned	$ 214,446	$ 215,788	$ 209,149	$ 208,149	$ 202,033	$ 206,070	$ 223,591	$ 187,159	847,532	818,853	738,531
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned									(81,738)	(88,235)	(87,760)
Net premiums earned									499,058	491,787	449,823
Premiums assumed under Quota Share Agreement									100	5,000	11,800
Specialty P&C | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									517,977	518,303	477,561
Specialty P&C | Legal professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									26,766	26,094	25,771
Specialty P&C | Medical technology liability
Segment Reporting Information [Line Items]
Gross premiums earned									33,957	35,157	33,836
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned									2,096	468	415
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Ceded premiums earned									(98,169)	(96,895)	(90,635)
Net premiums earned									189,240	186,079	163,309
Workers' Compensation Insurance | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned									203,195	199,466	173,246
Workers' Compensation Insurance | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned									84,214	83,508	80,698
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Ceded premiums earned									(9,741)	(9,324)	(8,714)
Net premiums earned									78,563	73,940	68,197
Segregated Portfolio Cell Reinsurance | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									6,059	5,009	4,097
Segregated Portfolio Cell Reinsurance | Other
Segment Reporting Information [Line Items]
Gross premiums earned									480	0	0
Segregated Portfolio Cell Reinsurance | Workers' compensation
Segment Reporting Information [Line Items]
Gross premiums earned									81,765	78,255	72,814
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Gross premiums earned									101,222	83,307	69,749
Ceded premiums earned									(20,551)	(16,260)	(12,547)
Net premiums earned									$ 80,671	$ 67,047	$ 57,202