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Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 17, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2012 | Dec. 31, 2018
Loss Contingencies [Line Items] |  |  |  |  |
Operating lease rent expense |  |  | $ 1,000 |  |
Litigation Matters |  |  |  |  |
Federal Reserve Bank reserve balance requirements included in cash and due from banks |  | $ 10,932 | $ 18,183 |  |
Visa Inc. |  |  |  |  |
Litigation Matters |  |  |  |  |
Settlement agreement amount |  |  |  | $ 4,000,000 |
Damages sought amended value | $ 4,100,000 |  |  |  |
Federal Reserve Bank reserve balance requirements included in cash and due from banks |  | $ 900,000 |  |  |
Minimum |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Weighted average remaining term (in years) |  | 1 year |  |  |
Finance lease, initial contract terms (in years) |  | 3 years |  |  |
Maximum |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Weighted average remaining term (in years) |  | 13 years |  |  |
Finance lease, initial contract terms (in years) |  | 5 years |  |  |
Total commitments and standby letters of credit | Interest payable and other liabilities |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Allowance for off balance sheet commitments |  | $ 1,087 |  |  | $ 958

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