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Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,168,885 |
Estimated Fair Value | 2,159,806 | $ 2,080,294
Corporate Debt Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 122,006 |
Estimated Fair Value | $ 121,288 |

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- Definition Amortized cost of 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 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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