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Fair Value Measurements of Investments Held by Retirement Plan (Parenthetical) (Detail) - Large-Cap Value - Common/Collective Trusts - Equity | Dec. 31, 2018 Stock
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Minimum |
Defined Benefit Plan Disclosure [Line Items] |
Number of stocks held | 60
Maximum |
Defined Benefit Plan Disclosure [Line Items] |
Number of stocks held | 70

X
- Definition Number of Stocks + References No definition available. + Details
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Namespace Prefix: | nyb_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- 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
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |