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Investments - Unrealized Losses and Fair Value for Mortgage-Backed Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Less than 12 months |  |
Gross Unrealized Losses | $ (1,893) | $ (849)
Estimated Fair Value | 48,128 | 82,279
12 months or more |  |
Gross Unrealized Losses | (7,614) | (3,320)
Estimated Fair Value | 120,893 | 124,618
Gross Unrealized Losses | (9,507) | (4,169)
Estimated Fair Value | 169,021 | 206,897
Mortgage-backed securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (1,768) | (772)
Estimated Fair Value | 43,253 | 77,356
12 months or more |  |
Gross Unrealized Losses | (6,278) | (2,438)
Estimated Fair Value | 104,841 | 110,500
Gross Unrealized Losses | (8,046) | (3,210)
Estimated Fair Value | 148,094 | 187,856
Utah Housing Corporation bonds |  |
Less than 12 months |  |
Gross Unrealized Losses | (125) | (77)
Estimated Fair Value | 4,875 | 4,923
12 months or more |  |
Gross Unrealized Losses | (1,336) | (882)
Estimated Fair Value | 16,052 | 14,118
Gross Unrealized Losses | (1,461) | (959)
Estimated Fair Value | $ 20,927 | $ 19,041

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- Definition Debt Securities, Available-for-sale, Unrealized Loss Position, More Than 12 Months [Abstract] + References No definition available. + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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