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Loss Reserves - Prior Year Loss Reserves (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior years | $ (231,204) | $ (147,658) | $ (110,302)
Decrease in estimated claim rate on primary delinquencies |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior years | (248,000) | (148,000) | (141,000)
Increase in estimated severity on primary delinquencies |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior years | 9,000 | 9,000 | 43,000
Change in estimates related to pool reserves, LAE reserves, reinsurance and other |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior years | $ 8,000 | $ (9,000) | $ (12,000)

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + 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 (b) -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 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 + Details
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