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Dispositions and Held-for-Sale (Held-for-Sale) (Details) $ in Thousands | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Nov. 11, 2017 banking_branch
--- | --- | --- | ---
Long Lived Assets Held-for-sale [Line Items] |  |  |
Branch assets held-for-sale | $ 0 | $ 91,318 |
Branch liability held-for-sale | $ 0 | 605,111 |
Desert Community Bank |  |  |
Long Lived Assets Held-for-sale [Line Items] |  |  |
Branch liability held-for-sale |  | 605,100 |
Desert Community Bank | California |  |  |
Long Lived Assets Held-for-sale [Line Items] |  |  |
Number of branches to be sold | banking_branch |  |  | 8
Loans held-for-sale | Desert Community Bank |  |  |
Long Lived Assets Held-for-sale [Line Items] |  |  |
Branch assets held-for-sale |  | 78,100 |
Premises and equipment held-for-sale | Desert Community Bank |  |  |
Long Lived Assets Held-for-sale [Line Items] |  |  |
Branch assets held-for-sale |  | $ 8,000 |

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- Definition Branch Assets Held-for-sale as part of the branch purchase agreement. + References No definition available. + Details
Name: | ewbc_BranchAssetsHeldForSale
Namespace Prefix: | ewbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Deposits Held-For-Sale + References No definition available. + Details
Name: | ewbc_DepositsHeldForSale
Namespace Prefix: | ewbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Number Of Branches To Be Sold + References No definition available. + Details
Name: | ewbc_NumberOfBranchesToBeSold
Namespace Prefix: | ewbc_
Data Type: | xbrli:integerItemType
Balance Type: | na
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: | us-gaap_LongLivedAssetsHeldForSaleLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=ewbc_DesertCommunityBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementGeographicalAxis=stpr_CA
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByAssetClassAxis=ewbc_LoansHeldForSaleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByAssetClassAxis=ewbc_PremisesAndEquipmentHeldForSaleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |