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Securities Securities - Amortized Cost, Allowance for Credit Losses and Estimated Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | $ 9,914,688 | $ 10,877,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 689,835 | 405,399
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 43 | 13,577
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 10,604,480 | 11,269,591
US Treasury Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 1,084,210 | 1,941,283
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 40,755 | 18,934
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 12,084
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 1,124,965 | 1,948,133
Residential Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 2,092,184 | 2,176,275
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 72,021 | 32,608
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 43 | 1,289
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 2,164,162 | 2,207,594
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 6,695,946 | 6,717,344
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 577,059 | 353,857
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 204
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 7,273,005 | 7,070,997
Other Investments [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 42,348 | 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, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | $ 42,348 | $ 42,867

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References No definition available. + Details
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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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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition Amount of allowance for credit loss 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 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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