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Retroactive Insurance (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Liabilities for Guarantees on Long-Duration Contracts [Line Items] |  |  |  |  |  |
Net premiums earned | $ 223,591 | $ 180,353 | $ 410,750 | $ 363,256 |  |
Net losses and loss adjustment expenses | 161,728 | 115,550 | 291,515 | 234,701 |  |
Reserve for losses and loss adjustment expenses | 2,078,817 | $ 1,991,865 | 2,078,817 | $ 1,991,865 | $ 2,048,381 | $ 1,993,428
Retroactive Insurance Contract |  |  |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts [Line Items] |  |  |  |  |  |
Net premiums earned | 26,600 |  | 26,600 |  |  |
Net losses and loss adjustment expenses | 25,400 |  | 25,400 |  |  |
Reserve for losses and loss adjustment expenses | 600 |  | 600 |  |  |
Retroactive Insurance Contract, Prospective Coverage |  |  |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts [Line Items] |  |  |  |  |  |
Net premiums earned | 7,900 |  | 7,900 |  |  |
Retroactive Insurance Contract, Retroactive Coverage |  |  |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts [Line Items] |  |  |  |  |  |
Net premiums earned | $ 18,700 |  | $ 18,700 |  |  |

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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- Definition Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred. + 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 210.7-04.5) -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 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums earned. + 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) -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 + Details
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