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Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Statement Of Cash Flows [Abstract] |  |  |  |
Amortization of operating lease right-of-use assets | $ 4.7 | $ 4.1 | $ 20.0 | $ 38.4
Amortization of operating lease liability | $ 4.7 | $ 4.1 | $ 20.0 | $ 38.4

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- Definition Amount of amortization expense attributable to operating lease liability. + References No definition available. + Details
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Namespace Prefix: | nycb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of amortization expense for right-of-use asset from operating lease. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAssetAmortizationExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_StatementOfCashFlowsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration