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Financial Information - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Product Information [Line Items] |  |  |  |
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12 |  |  | $ 67 | $ 0
Revenue, performance obligation, description of timing | one year or less |  |  |
Available-for-sale Securities | Accounting Standards Update 2016-01 |  |  |  |
Product Information [Line Items] |  |  |  |
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12 |  | $ (265,900) |  |
Other Assets | Accounting Standards Update 2016-01 |  |  |  |
Product Information [Line Items] |  |  |  |
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12 |  | $ 265,900 |  |
Non-Interest income |  |  |  |
Product Information [Line Items] |  |  |  |
Accounts receivable | $ 9,000 |  |  |
Pro Forma | Accounting Standards Update 2016-02 |  |  |  |
Product Information [Line Items] |  |  |  |
Lease assets | 185,000 |  |  |
Lease liabilities | 195,000 |  |  |
Pro Forma | Retained Earnings | Accounting Standards Update 2016-02 |  |  |  |
Product Information [Line Items] |  |  |  |
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12 | $ 3,000 |  |  |

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