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Dividend Restrictions - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
Condensed Financial Statements, Captions [Line Items] |
Dividend paid to the Parent Company | $ 380.0
Additional dividends that could have been paid to the Parent Company without regulatory approval | $ 463.4

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- Definition The additional amount of dividends that may be paid or declared in any calendar year without prior approval of regulatory agencies. + References No definition available. + Details
Name: | nyb_AdditionalAmountAvailableForDividendDistributionWithoutPriorApprovalFromRegulatoryAgency
Namespace Prefix: | nyb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | srt_CondensedFinancialStatementsCaptionsLineItems
Namespace Prefix: | srt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50% or less owned persons accounted for using the equity method. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04(b)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690 + Details
Name: | us-gaap_CashDividendsPaidToParentCompany
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration