Segment Information - Gross Premiums Earned by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Ceded premiums earned									$ (110,347)	$ (95,315)	$ (101,510)
Net premiums earned	$ 182,972	$ 192,303	$ 180,353	$ 182,903	$ 193,694	$ 185,275	$ 176,732	$ 177,579	738,531	733,281	694,149
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned									(83,662)	(78,115)	(84,805)
Net premiums earned									453,921	457,816	443,313
Premiums ceded to Lloyd's Syndicate									11,800	14,000	14,400
Specialty P&C | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									477,561	474,981	463,599
Specialty P&C | Legal professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									25,771	26,125	28,234
Specialty P&C | Medical technology liability
Segment Reporting Information [Line Items]
Gross premiums earned									33,836	34,158	34,838
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned									415	667	1,447
Workers' Compensation
Segment Reporting Information [Line Items]
Ceded premiums earned									(25,893)	(25,335)	(25,122)
Net premiums earned									227,408	220,815	213,161
Workers' Compensation | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned									172,603	170,492	172,115
Workers' Compensation | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned									80,698	75,658	66,168
Lloyd's Syndicate
Segment Reporting Information [Line Items]
Ceded premiums earned									(12,547)	(5,914)	(5,942)
Net premiums earned									57,202	54,650	37,675
Lloyd's Syndicate | Property and casualty
Segment Reporting Information [Line Items]
Gross premiums earned									$ 69,749	$ 60,564	$ 43,617