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Debt Securities - Credit Quality Indictors for HTM Debt Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
HTM debt securities |  |
Debt securities, held-to-maturity | $ 588,751 | $ 189,836
State and local governments |  |
HTM debt securities |  |
Debt securities, held-to-maturity | 588,751 | 189,836
State and local governments | Moody's: Aaa | S&P: AAA | Investment Grade |  |
HTM debt securities |  |
Debt securities, held-to-maturity | 91,831 | 39,022
State and local governments | Moody's: Aa1, Aa2, Aa3 | S&P: AA+, AA, AA- | Investment Grade |  |
HTM debt securities |  |
Debt securities, held-to-maturity | 471,159 | 123,664
State and local governments | Moody's: A1, A2, A3 | S&P: A+, A, A- | Investment Grade |  |
HTM debt securities |  |
Debt securities, held-to-maturity | $ 25,761 | $ 27,150

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- Definition Amount of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: 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 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563 Reference 3: 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 + Details
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