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Reinsurance - Narrative (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jul. 01, 2015 | Mar. 31, 2018 | Sep. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |
Ceded premiums written, net of profit commission |  |  |  | $ 125,726,000 | $ 133,885,000 | $ 55,391,000 |
Ceded to Other Companies |  |  |  | 125,735,000 | 133,981,000 | 102,848,000 |
Reinsurance recoverable on loss reserves |  |  |  | $ 48,474,000 | $ 50,493,000 | 44,487,000 | $ 57,841,000
Period of existing captive reinsurance agreement |  |  |  | 10 years | 10 years |  |
Fair value of trust fund assets under our captive agreements |  |  |  | $ 80,000,000 | $ 91,000,000 |  |
Quota Share Reinsurance Agreement, 2017 |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |
Contingent termination fee |  |  |  | $ 0 |  |  |
Threshold for private mortgage insurer eligibility requirements for termination election |  |  |  | 90.00% |  |  |
Quota share, percentage |  |  |  | 30.00% |  |  |
Ceding commission, percentage |  |  |  | 20.00% |  |  |
Loss ratio |  |  |  | 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 |  |  |  | 90.00% |  |  |
Quota share, percentage |  |  |  | 30.00% |  |  |
Ceding commission, percentage |  |  |  | 20.00% |  |  |
Loss ratio |  |  |  | 60.00% |  |  |
Quota Share Reinsurance Agreement, 2013 |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |
Ceded premiums written, net of profit commission |  |  | $ 69,400,000 |  |  |  |
Ceded to Other Companies |  |  | 11,600,000 |  |  |  |
Ceding commissions |  |  | $ 11,600,000 |  |  |  |
Profit commission | $ 142,500,000 |  |  |  |  |  |
Quota Share Reinsurance Agreement, 2018 | Forecast |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |
Contingent termination fee |  | $ 0 |  |  |  |  |
Threshold for private mortgage insurer eligibility requirements for termination election |  | 90.00% |  |  |  |  |
Quota share, percentage |  | 30.00% |  |  |  |  |
Ceding commission, percentage |  | 20.00% |  |  |  |  |
Loss ratio |  | 54.00% |  |  |  |  |
Quota Share Reinsurance Agreements, Excluding Captives |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |
Ceded premiums written, net of profit commission |  |  |  | $ 120,974,000 | 125,460,000 | 41,233,000 |
Ceded to Other Companies |  |  |  | 120,974,000 | 125,460,000 | 88,587,000 |
Ceding commissions |  |  |  | 49,321,000 | 47,629,000 | 30,816,000 |
Profit commission |  |  |  | 125,629,000 | 112,685,000 | $ 112,847,000 |
Reinsurance recoverable on loss reserves |  |  |  | 39,300,000 | 31,800,000 |  |
Reinsurance Agreements, Captive Agreements |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |
Reinsurance recoverable on loss reserves |  |  |  | $ 9,000,000 | $ 19,000,000 |  |

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- Definition Amount of fees and commissions from financial services and banking activities and correspondent clearing. Includes fees from depositor accounts, credit cards, merchant discounts, and fiduciary and trust activities, guarantee fees, investment advisory, management, and administrative fees, transfer agent fees, and insurance commissions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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