Segment Information - Gross Premiums Earned by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Ceded premiums earned									$ (95,315)	$ (101,510)	$ (68,879)
Net premiums earned	$ 193,694	$ 185,275	$ 176,732	$ 177,579	$ 164,874	$ 182,085	$ 175,293	$ 171,899	733,281	694,149	699,731
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned									(78,115)	(84,805)	(50,319)
Net premiums earned									457,816	443,313	492,733
Premiums ceded to Lloyd's Syndicate									14,000	14,400	4,200
Specialty P&C | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									474,981	463,599	477,031
Specialty P&C | Legal professional liability
Segment Reporting Information [Line Items]
Gross premiums earned									26,125	28,234	28,278
Specialty P&C | Medical technology liability
Segment Reporting Information [Line Items]
Gross premiums earned									34,158	34,838	35,913
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned									667	1,447	1,830
Workers' Compensation
Segment Reporting Information [Line Items]
Ceded premiums earned									(25,335)	(25,122)	(21,793)
Net premiums earned									220,815	213,161	194,540
Workers' Compensation | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned									170,492	172,115	160,717
Workers' Compensation | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned									75,658	66,168	55,616
Lloyd's Syndicate
Segment Reporting Information [Line Items]
Ceded premiums earned									(5,914)	(5,942)	(971)
Net premiums earned									54,650	37,675	12,458
Lloyd's Syndicate | Property and casualty
Segment Reporting Information [Line Items]
Gross premiums earned									$ 60,564	$ 43,617	$ 13,429