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Segment Information (Gross Premiums Earned by Product and Reconciliation to Net Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 223,591	$ 180,353	$ 410,750	$ 363,256
Specialty P&C
Segment Reporting Information [Line Items]
Ceded premiums earned	(17,749)	(21,683)	(35,169)	(40,407)
Net premiums earned	143,847	109,005	260,125	222,063
Specialty P&C | Healthcare professional liability
Segment Reporting Information [Line Items]
Gross premiums earned	146,208	115,781	264,892	232,832
Specialty P&C | Legal professional liability
Segment Reporting Information [Line Items]
Gross premiums earned	6,489	6,417	12,880	12,734
Specialty P&C | Medical technology liability
Segment Reporting Information [Line Items]
Gross premiums earned	8,780	8,389	17,292	16,701
Specialty P&C | Other
Segment Reporting Information [Line Items]
Gross premiums earned	119	101	230	203
Workers' Compensation
Segment Reporting Information [Line Items]
Ceded premiums earned	(7,026)	(5,856)	(14,031)	(11,787)
Net premiums earned	62,254	56,854	120,660	112,137
Workers' Compensation | Traditional business
Segment Reporting Information [Line Items]
Gross premiums earned	48,632	42,501	94,662	84,269
Workers' Compensation | Alternative market business
Segment Reporting Information [Line Items]
Gross premiums earned	20,648	20,209	40,029	39,655
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Ceded premiums earned	(1,783)	(2,466)	(7,275)	(5,089)
Net premiums earned	17,490	14,494	29,965	29,056
Premiums assumed under Quota Share Agreement	1,400	3,100	3,300	6,600
Lloyd's Syndicates | Property and casualty
Segment Reporting Information [Line Items]
Gross premiums earned	$ 19,273	$ 16,960	$ 37,240	$ 34,145

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.1)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b),(c)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84167274&loc=d3e27175-158546

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17)
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

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-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
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-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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