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Dividend Restrictions - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($)
Condensed Financial Statements, Captions [Line Items] |
Dividend paid to the Parent Company | $ 336.0
Additional dividends that could have been paid to the Parent Company without regulatory approval | $ 379.5

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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 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/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.9-06) -URI http://asc.fasb.org/extlink&oid=6882215&loc=d3e537971-122884 + Details
Name: | us-gaap_CashDividendsPaidToParentCompany
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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: | us-gaap_CondensedFinancialStatementsCaptionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration