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Securities - Contractual Maturities of Debt Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |
Debt Securities, Amortized Cost | $ 690,251 |  |
Debt Securities, Fair Value | 682,454 | $ 693,147 |
Obligations of the U.S. Treasury, U.S. government corporations and agencies, and obligations of states and political subdivisions |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Due in one year or less, Amortized Cost | 43,472 |  |
Due after one year through five years, Amortized Cost | 148,616 |  |
Due after five years through ten years, Amortized Cost | 78,565 |  |
Due after ten years, Amortized Cost | 28,792 |  |
Debt Securities, Amortized Cost | 299,445 |  |
Due in one year or less, Fair Value | 43,372 |  |
Due after one year through five years, Fair Value | 148,787 |  |
Due after five years through ten years, Fair Value | 78,767 |  |
Due after ten years, Fair Value | 29,465 |  |
Debt Securities, Fair Value | 300,391 |  |
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, 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, 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, Fair Value | $ 248,798 | $ 242,221 | [1]
[1] | Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

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