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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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- Definition Amount of earned premiums ceded to other entities. + References 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 + Details
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