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Investments in Securities - Additional Information (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Marketable Securities [Line Items] |  |
Securities classified as held to maturity | $ 0.0 | $ 0.0
Fair Value of Debt Securities Reported Less Than Their Historical Cost | $ 24.2 | $ 116.2
Debt Securities Reported Less Than Their Historical Cost Percent of Investment Portfolio | 5.80% | 31.10%
Fixed rates of interest |  |
Marketable Securities [Line Items] |  |
Agency collateralized mortgage obligations | $ 156.8 |
Mortgage Backed Securities | 169.9 |
Variable rates of interest |  |
Marketable Securities [Line Items] |  |
Agency collateralized mortgage obligations | 19.8 |
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |
Marketable Securities [Line Items] |  |
Agency collateralized mortgage obligations | 52.9 |
Agency mortgage-backed securities | 156.6 |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] |  |
Marketable Securities [Line Items] |  |
Agency collateralized mortgage obligations | 17.9 |
Agency mortgage-backed securities | 10.3 |
Government National Mortgage Association Certificates and Obligations (GNMA) |  |
Marketable Securities [Line Items] |  |
Agency collateralized mortgage obligations | 105.8 |
Agency mortgage-backed securities | $ 3.0 |

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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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