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Securities - Fair Values and Amortized Costs Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | $ 690,251 |  |
Debt Securities, Gross Unrealized Gains |  | 3,070 | $ 5,645 |
Debt Securities, Gross Unrealized Losses |  | (10,867) | (3,967) |
Debt Securities, Fair Value |  | 682,454 | 693,147 |
Equity Securities, Fair Value |  | 5,196 | 5,144 |
Total Securities, Amortized Cost |  |  | 695,284 |
Total Securities, Gross Unrealized Gains |  |  | 6,975 |
Total Securities, Gross Unrealized Losses |  |  | (3,968) |
Total Securities, Fair Value |  | 687,650 | 698,291 |
U.S. Treasury securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 14,951 | 19,943 |
Debt Securities, Gross Unrealized Gains |  | 0 | 0 |
Debt Securities, Gross Unrealized Losses |  | (290) | (154) |
Debt Securities, Fair Value |  | 14,661 | 19,789 |
Obligations of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 161,844 | 162,045 |
Debt Securities, Gross Unrealized Gains |  | 62 | 341 |
Debt Securities, Gross Unrealized Losses |  | (1,317) | (193) |
Debt Securities, Fair Value |  | 160,589 | 162,193 |
Collateralized mortgage obligations of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 105,063 | 109,916 |
Debt Securities, Gross Unrealized Gains |  | 2 | 93 |
Debt Securities, Gross Unrealized Losses |  | (2,483) | (1,321) |
Debt Securities, Fair Value |  | 102,582 | 108,688 |
Residential mortgage-backed securities of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 30,700 | 32,388 |
Debt Securities, Gross Unrealized Gains |  | 443 | 679 |
Debt Securities, Gross Unrealized Losses |  | (460) | (213) |
Debt Securities, Fair Value |  | 30,683 | 32,854 |
Commercial mortgage-backed securities of U.S. government corporations and agencies |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 255,043 | 244,018 | [1]
Debt Securities, Gross Unrealized Gains |  | 0 | 247 | [1]
Debt Securities, Gross Unrealized Losses |  | (6,245) | (2,044) | [1]
Debt Securities, Fair Value |  | 248,798 | 242,221 | [1]
Securities purchased pending settlement |  |  | 5,900 |
Obligations of states and political subdivisions |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 122,650 | 123,159 |
Debt Securities, Gross Unrealized Gains |  | 2,563 | 4,285 |
Debt Securities, Gross Unrealized Losses |  | (72) | (42) |
Debt Securities, Fair Value |  | 125,141 | 127,402 |
Total Debt Securities Available-for-Sale |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Debt Securities, Amortized Cost |  | 690,251 | 691,469 |
Debt Securities, Gross Unrealized Gains |  | 3,070 | 5,645 |
Debt Securities, Gross Unrealized Losses |  | (10,867) | (3,967) |
Debt Securities, Fair Value |  | 682,454 | 693,147 |
Total equity securities |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Equity Securities, FV-NI, Cost | [2] | 0 | 3,815 |
Equity Securities, FV-NI, Unrealized Gain | [2] | 0 | 1,330 |
Equity Securities, FV-NI, Unrealized Loss | [2] | 0 | (1) |
Equity Securities, Fair Value | [2] | $ 0 | $ 5,144 |
[1] | Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.
[2] | ASU 2016-01 was adopted January 1, 2018, resulting in separate classification of our marketable equity securities previously included in available-for-sale securities

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