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Reinsurance - Narrative (Details) - USD ($) | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016
Effects of Reinsurance [Line Items] |  |  |  |
Reinsurance recoverable on loss reserves | $ 33,281,000 | $ 48,474,000 | $ 45,878,000 | $ 50,493,000
Quota Share Reinsurance Agreement, 2018 |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Contingent termination fee | $ 0 |  |  |
Threshold for private mortgage insurer eligibility requirements for termination election (less than) (as a percent) | 90.00% |  |  |
Quota share for all policies covered (as a percent) | 30.00% |  |  |
Ceding commission, percentage (as a percent) | 20.00% |  |  |
Loss ratio threshold for profit commissions (as a percent) | 62.00% |  |  |
Quota Share Reinsurance Agreement, 2017 |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Contingent termination fee | $ 0 |  |  |
Threshold for private mortgage insurer eligibility requirements for termination election (less than) (as a percent) | 90.00% |  |  |
Quota share for all policies covered (as a percent) | 30.00% |  |  |
Ceding commission, percentage (as a percent) | 20.00% |  |  |
Loss ratio threshold for profit commissions (as a percent) | 60.00% |  |  |
Quota Share Reinsurance Agreement, 2015 |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Contingent termination fee | $ 0 |  |  |
Threshold for private mortgage insurer eligibility requirements for termination election (less than) (as a percent) | 90.00% |  |  |
Quota share for all policies covered (as a percent) | 30.00% |  |  |
Ceding commission, percentage (as a percent) | 20.00% |  |  |
Loss ratio threshold for profit commissions (as a percent) | 60.00% |  |  |
Quota Share Reinsurance Agreements, Excluding Captive Agreements |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Reinsurance recoverable on loss reserves | $ 33,100,000 | $ 39,300,000 |  |

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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + 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 (cc) -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 (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 + Details
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