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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 | Jun. 30, 2021 | Dec. 31, 2020
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Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | $ 10,259,473 | $ 9,727,401
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 593,881 | 710,168
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 12,828 | 4
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 10,840,526 | 10,437,565
US Treasury Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 1,473,800 | 1,084,542
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 26,664 | 35,091
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 2,742 | 0
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 1,497,722 | 1,119,633
Residential Mortgage Backed Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 1,729,198 | 1,916,581
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 40,589 | 71,102
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 8,158 | 4
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 1,761,629 | 1,987,679
US States and Political Subdivisions Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 7,014,119 | 6,683,927
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax | 526,628 | 603,975
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 1,928 | 0
Debt Securities, Held-to-maturity, Allowance for Credit Loss | 0 | 0
Securities available for sale, estimated fair value | 7,538,819 | 7,287,902
Other Investments [Member] |  |
Schedule of Investments [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 42,356 | 42,351
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 | $ 42,356 | $ 42,351

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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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