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Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | $ 9,727,401 | $ 10,877,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 710,168 | 405,399
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 4 | 13,577
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available for Sale, Estimated Fair Value | 10,437,565 | 11,269,591
US Treasury [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 1,084,542 | 1,941,283
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 35,091 | 18,934
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 12,084
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available for Sale, Estimated Fair Value | 1,119,633 | 1,948,133
Residential Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 1,916,581 | 2,176,275
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 71,102 | 32,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 4 | 1,289
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available for Sale, Estimated Fair Value | 1,987,679 | 2,207,594
States and Political Subdivisions [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 6,683,927 | 6,717,344
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 603,975 | 353,857
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 204
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available for Sale, Estimated Fair Value | 7,287,902 | 7,070,997
Other [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 42,351 | 42,867
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available for Sale, Estimated Fair Value | $ 42,351 | $ 42,867

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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