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Securities - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2007
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Gross gains | $ 295 |  | $ 0 | $ 3,600 |
Debt Securities, Available-for-sale, Realized Loss | 0 |  | $ 0 |  |
Collateral Pledged [Member] |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Debt Securities, Available-for-sale, Restricted | $ 2,600 | $ 2,700 |  | 2,700 |
Other Investments [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investments, Fair Value Disclosure |  | 3,427 |  | 3,427 |
Fair Value, Inputs, Level 3 [Member] | Other Investments [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Investments, Fair Value Disclosure |  | $ 3,427 |  | $ 3,427 |
Common Class B [Member] |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Sale of Stock, Number of Shares Issued in Transaction |  | (25,702) |  |  |
Visa Inc's Initial Public Offering [Member] | Common Class B [Member] |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Sale of Stock, Number of Shares Issued in Transaction |  |  |  |  | (51,404)
Shares, Outstanding |  | (25,702) |  | (25,702) |

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- Definition Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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