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Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Less than 12 months, Gross Unrealized Losses | $ (849) | $ (3,113)
12 months or more, Gross Unrealized Losses | (3,320) | (619)
Total, Gross Unrealized Losses | (4,169) | (3,732)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months, Estimated Fair Value | 82,279 | 143,859
12 months or more, Estimated Fair Value | 124,618 | 10,885
Total, Estimated Fair Value | 206,897 | 154,744
Mortgage-backed securities |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Less than 12 months, Gross Unrealized Losses | (772) | (2,423)
12 months or more, Gross Unrealized Losses | (2,438) | (619)
Total, Gross Unrealized Losses | (3,210) | (3,042)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months, Estimated Fair Value | 77,356 | 129,549
12 months or more, Estimated Fair Value | 110,500 | 10,885
Total, Estimated Fair Value | 187,856 | 140,434
Utah Housing Corporation bonds |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract] |  |
Less than 12 months, Gross Unrealized Losses | (77) | (690)
12 months or more, Gross Unrealized Losses | (882) | 0
Total, Gross Unrealized Losses | (959) | (690)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months, Estimated Fair Value | 4,923 | 14,310
12 months or more, Estimated Fair Value | 14,118 | 0
Total, Estimated Fair Value | $ 19,041 | $ 14,310

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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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