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Financial Information (Narrative) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | --- | ---
Product Information [Line Items] |  |  |  |
Cumulative effect of new accounting principle in period of adoption |  |  | $ (67) | $ 0
Available-for-sale Securities | Accounting Standards Update 2016-01 |  |  |  |
Product Information [Line Items] |  |  |  |
Cumulative effect of new accounting principle in period of adoption |  | $ 265,900 |  |
Other Assets | Accounting Standards Update 2016-01 |  |  |  |
Product Information [Line Items] |  |  |  |
Cumulative effect of new accounting principle in period of adoption |  | $ (265,900) |  |
Non-Interest income |  |  |  |
Product Information [Line Items] |  |  |  |
Accounts receivable | $ 7,700 |  |  |

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