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Investment Securities - Amortized Cost and Estimated Fair Value of Available for Sale Securities (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
Other equity securities | $ 2,978 | $ 3,414 | $ 3,327
Securities available for sale, Amortized Cost | 314,219 |  |
Securities available for sale, Unrealized Gains | 144 |  |
Securities available for sale, Unrealized Losses | (9,315) |  |
Securities available for sale | 305,048 |  |
Amortized Cost |  | 303,845 | 309,046
Unrealized Gains |  | 859 | 1,932
Unrealized Losses |  | (4,532) | (2,832)
Fair Value (Estimated) |  | 300,172 | 308,146
Mortgage-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 306,286 |  |
Unrealized Gains | 144 |  |
Unrealized Losses | (9,052) |  |
Fair Value (Estimated) | 297,378 |  |
Amortized Cost |  | 293,689 | 290,379
Unrealized Gains |  | 722 | 1,381
Unrealized Losses |  | (4,422) | (2,769)
Fair Value (Estimated) |  | 289,989 | 288,991
State and political subdivisions |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 4,955 |  |
Unrealized Gains | 0 |  |
Unrealized Losses | (263) |  |
Fair Value (Estimated) | $ 4,692 |  |
Amortized Cost |  | 6,860 | 15,401
Unrealized Gains |  | 16 | 486
Unrealized Losses |  | (107) | (59)
Fair Value (Estimated) |  | 6,769 | 15,828
Other equity securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost |  | 3,296 | 3,266
Unrealized Gains |  | 121 | 65
Unrealized Losses |  | (3) | (4)
Fair Value (Estimated) |  | $ 3,414 | $ 3,327

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