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Reinsurance - Narrative (Details) - USD ($) | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017 | Dec. 31, 2016
Effects of Reinsurance [Line Items] |  |  |  |
Reinsurance recoverable on loss reserves | $ 45,474,000 | $ 48,474,000 | $ 46,658,000 | $ 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 | $ 43,500,000 | $ 39,300,000 |  |

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