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Investment Securities - Narrative (Details) - USD ($) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Investment Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amount Outstanding | $ 3,113,742,000 |  | $ 2,226,409,000
Fair Value | $ 3,111,632,000 |  | $ 2,217,852,000
Average percent of amortized cost for securities in unrealized loss positions for more than 12 months | 99.20% |  |
Other-than-temporary impairment | $ 0 | $ 0 |
Floating Interest Rate [Member] | Corporate Debt Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amount Outstanding | 1,700,000 |  |
Fair Value | $ 1,600,000 |  |

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