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Investment Securities - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sale of available-for-sale investment securities | $ 10,218 | $ 186,769 | $ 83,735
Available-for-sale Securities, Gross Realized Gains | 100 | 100 | 100
Available-for-sale Securities, Gross Realized Losses | $ 0 | $ 100 | $ 0
Fixed Interest Rate [Member] | Government/GSE Guaranteed Mortgage Backed Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Held-to-Maturity Securities, Debt Maturities, Weighted-Average Yield | 2.50% |  |

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