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Investment Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items] |  |  |
Held-to-maturity, interest receivable | $ 1,000 |  |
Available-for-sale, interest receivable | 17,100 |  |
Estimated Fair Value | 3,473,598 | $ 3,288,343 |
Securities valued at less than historical cost, amount | $ 559,281 |  |
Securities continuous unrealized loss position fair value (as percent) | 16.10% |  |
Decrease in available-for-sale provision for credit losses | $ (61) | 0 | $ 0
Securities pledged as collateral | 2,000,000 | 1,700,000 |
Realized gains | 54,800 | 13,300 | 65
Realized losses | $ 15 | $ 4 | $ 4
Income tax benefit related to security gains (losses) (as percent) | 26.135% | 26.135% | 26.135%
Bonds |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Investments sold, amount | $ 1,700,000 |  |

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- Definition Amount 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: 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 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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