Summary of Significant Accounting Policies and Nature of Operations (Policies)	9 Months Ended
Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	The unaudited Consolidated Financial Statements of MUFG Americas Holdings Corporation, its subsidiaries, and its consolidated variable interest entities (the Company) have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP) for interim financial reporting and the instructions to Form 10-Q and Rule 10-01 of Regulation S-X of the Rules and Regulations of the SEC. However, they do not include all of the disclosures necessary for annual financial statements. In the opinion of management, all adjustments, consisting of normal recurring accruals, considered necessary for a fair presentation have been included.
Use of Estimates	The preparation of financial statements in conformity with GAAP also requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expense during the reporting period. Although such estimates contemplate current conditions and management’s expectations of how they may change in the future, it is reasonably possible that actual results could differ significantly from those estimates. This could materially affect the Company’s results of operations and financial condition in the near term. Critical estimates made by management in the preparation of the Company’s financial statements include, but are not limited to, the allowance for credit losses (Note 3), goodwill impairment, fair value of financial instruments (Note 8), hedge accounting (Note 9), pension accounting (Note 11), income taxes, and transfer pricing.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements Revenue from Contracts with Customers In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers, which provides guidance on the core principle that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This ASU applies to all contracts with customers, except financial instruments, guarantees, lease contracts, insurance contracts and certain non-monetary exchanges. It provides the following five-step revenue recognition model: (1) identify the contract(s) with a customer, (2) identify the performance obligations in the contract, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations in the contract, and (5) recognize revenue when (or as) the entity satisfies a performance obligation. In addition, the ASU requires additional disclosure of the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. In August 2015, the FASB issued ASU 2015-14, Deferral of the Effective Date, which deferred the effective date of ASU 2014-09 to interim and annual periods beginning on January 1, 2018, with early adoption permitted in 2017. The Company plans to apply the modified retrospective method upon adoption on January 1, 2018. As part of our implementation progress to date, we are in the implementation phase and are evaluating potential changes to processes and controls. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations. Recognition and Measurement of Financial Assets and Financial Liabilities In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities, which amends the accounting, presentation, and disclosure requirements for certain financial instruments. The ASU requires that all equity investments be recorded at fair value through net income (other than those accounted for under equity method or result in consolidation of the investee); however, an entity may choose to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. The ASU also requires an entity to present separately in other comprehensive income the portion of the total change in fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability under the fair value option. The ASU also clarifies that an entity must evaluate the need for a valuation allowance on a deferred tax asset related to available for sale securities in combination with the entity’s other deferred tax assets. In addition, the ASU amends the presentation and disclosure requirements for financial instruments and now requires the use of an exit price notion when measuring the fair value of financial instruments for disclosure purposes. The ASU is effective for interim and annual periods beginning on January 1, 2018, with early adoption permitted for the amendments to the accounting for financial liabilities under the fair value option. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations. Accounting for Leases In February 2016, the FASB issued ASU 2016-02, Leases, which will require entities that lease assets (i.e., lessees) to recognize assets and liabilities on their balance sheet for the rights and obligations created by those leases. The accounting by entities that own the assets leased (i.e., lessors) will remain largely unchanged; however, leveraged lease accounting will no longer be permitted for leases that commence after the effective date. The ASU will also require qualitative and quantitative disclosures about the amount, timing and uncertainty of cash flows arising from leases. The ASU is effective for interim and annual periods beginning on January 1, 2019 and requires a modified retrospective approach, with early adoption permitted. The Company plans to adopt the ASU on January 1, 2019. The Company recently began the technology design phase of this project to support the ongoing lessee accounting required under the ASU. Management is currently assessing the impact of this guidance on the Company's financial position and results of operations. Principal versus Agent Considerations (Reporting Revenue Gross versus Net) In March 2016, the FASB issued ASU 2016-08, Principal versus Agent Considerations (Reporting Revenue Gross versus Net). The ASU amends ASU 2014-09, Revenue from Contracts with Customers, with respect to assessing whether an entity is a principal (and thus presents revenue gross) or an agent (and thus presents revenue net). The amendments retain the guidance that the principal in an arrangement controls a good or service before it is transferred to a customer and clarify: (1) that an entity must first identify the specified good or service being provided to the customer; (2) that the unit of account for the principal versus agent assessment is each specified good or service promised in a contract; (3) indicators and examples to help an entity evaluate whether it is the principal; and (4) how to assess whether an entity controls services performed by another party. The ASU is effective upon the adoption of ASU 2014-09, which the Company plans to adopt beginning January 1, 2018, with early adoption permitted in 2017. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations. Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments, which provides new guidance on the accounting for credit losses for instruments that are within its scope. For loans and debt securities accounted for at amortized cost, certain off-balance sheet credit exposures, net investments in leases, and trade receivables, the ASU requires an entity to recognize its estimate of credit losses expected over the life of the financial instrument or exposure. Lifetime expected credit losses on purchased financial assets with credit deterioration will be recognized as an allowance with an offset to the cost basis of the asset. For available for sale debt securities, the new standard will require recognition of expected credit losses by recognizing an allowance for credit losses when the fair value of the security is below amortized cost and the recognition of this allowance is limited to the difference between the security’s amortized cost basis and fair value. The ASU is effective for interim and annual periods beginning on January 1, 2020, with early adoption permitted in 2019. The Company plans to adopt the ASU on January 1, 2020. The Company is currently collecting business and data requirements to support the project planning phase of the implementation. Management is currently assessing the impact of this guidance on the Company’s financial position and results of operations. Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments, to address diversity in practice in how certain cash receipts and payments are presented and classified in the statement of cash flows. The ASU is effective for interim and annual periods beginning on January 1, 2018, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company's statement of cash flows. Income Tax Consequences of Intra-Entity Asset Transfers In October 2016, the FASB issued ASU 2016-16, Intra-Entity Transfers of Assets Other Than Inventory, to improve the accounting for the income tax consequences of intra-entity assets other than inventory. The ASU will require recognition of the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The ASU is effective for interim and annual periods beginning on January 1, 2018, with early adoption permitted. Management does not expect adoption of this guidance to significantly impact the Company's financial position and results of operations. Restricted Cash In November 2016, the FASB issued ASU 2016-18, Restricted Cash, to address diversity in the classification and presentation of changes in restricted cash on the statement of cash flows. The ASU requires that a statement of cash flows explains the change during the period in the total of cash, cash equivalents, and the amounts generally described as restricted cash or restricted cash equivalents. The ASU is effective for interim and annual periods beginning on January 1, 2018 using a retrospective transition method. Early adoption is permitted. Management does not expect the adoption of this guidance to have a significant impact on the Company's statement of cash flows. Clarifying the Definition of a Business In January 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business, which specifies when a set of assets and activities constitutes a business. The ASU adds a “screen” to determine when a set is not a business, thus reducing the number of transactions deemed businesses. Specifically, if the fair value of the gross assets acquired is concentrated in a single identifiable asset (or a group of similar identifiable assets), the set is not deemed a business. Otherwise, to be considered a business, a set must include at least one input and a substantive process that together significantly contribute to the ability to create outputs. Although outputs are not required to be a business, the ASU narrows the definition of an output and limits the instances where sets that lack outputs are deemed businesses. The ASU is effective for interim and annual periods prospectively beginning on January 1, 2018, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations. Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued ASU 2017-04, Simplifying the Test for Goodwill Impairment. The ASU removes Step 2 of the goodwill impairment test, which measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. Under the amendments, a goodwill impairment loss will be measured as the amount by which a reporting unit’s carrying amount exceeds its fair value; however, the loss recognized cannot exceed the total amount of goodwill allocated to that reporting unit. Additionally, an entity should consider income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit when measuring the goodwill impairment loss. The ASU will be effective for MUAH beginning January 1, 2020 on a prospective basis. Early adoption is permitted for any impairment tests performed after January 1, 2017. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position or results of operations. Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets In February 2017, the FASB issued ASU 2017-05, Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets, to clarify the scope of ASC 610-20, Other Income - Gains and Losses from Derecognition of Nonfinancial Assets (issued as part of ASU 2014-09), and provide guidance on partial sales of nonfinancial assets. The ASU clarifies that the unit of account under ASC 610-20 is each distinct nonfinancial or in substance nonfinancial asset and that a financial asset that meets the definition of an “in substance nonfinancial asset” is within the scope of ASC 610-20. The ASU eliminates rules specifically addressing sales of real estate and removes exceptions to the financial asset derecognition model. The ASU is effective upon the adoption of ASU 2014-09, which the Company plans to adopt beginning January 1, 2018. It allows an entity to use either a retrospective or modified retrospective approach. Management does not expect the adoption of this guidance to significantly impact the Company's financial position and results of operations. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued ASU 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, which amends the income statement presentation of the components of net periodic benefit cost for sponsored defined benefit pension and other postretirement plans. The ASU also now mandates that only the service cost component of net benefit cost is eligible for capitalization on certain internally produced assets. The ASU is effective for interim and annual periods beginning January 1, 2018, with retrospective application for the new income statement presentation requirements and prospective application for the new capitalization requirement. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations. Premium Amortization on Purchased Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities, which requires premiums on certain purchased callable debt securities to be amortized to the earliest call date. Under current guidance, premiums on callable debt securities are generally amortized over the contractual life of the security. The amortization period for callable debt securities purchased at a discount will not be impacted. The ASU is effective for interim and annual periods beginning on January 1, 2019 and requires a modified retrospective approach, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations. Scope of Modification Accounting In May 2017, the FASB issued ASU 2017-09, Scope of Modification Accounting, to provide guidance about which changes to the terms and conditions of a share-based payment award require an entity to apply modification accounting. The ASU is effective for interim and annual periods beginning January 1, 2018, and will be applied prospectively, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Recently Issued Accounting Pronouncements

Revenue from Contracts with Customers

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers, which provides guidance on the core principle that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This ASU applies to all contracts with customers, except financial instruments, guarantees, lease contracts, insurance contracts and certain non-monetary exchanges. It provides the following five-step revenue recognition model: (1) identify the contract(s) with a customer, (2) identify the performance obligations in the contract, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations in the contract, and (5) recognize revenue when (or as) the entity satisfies a performance obligation. In addition, the ASU requires additional disclosure of the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. In August 2015, the FASB issued ASU 2015-14, Deferral of the Effective Date, which deferred the effective date of ASU 2014-09 to interim and annual periods beginning on January 1, 2018, with early adoption permitted in 2017. The Company plans to apply the modified retrospective method upon adoption on January 1, 2018. As part of our implementation progress to date, we are in the implementation phase and are evaluating potential changes to processes and controls. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Recognition and Measurement of Financial Assets and Financial Liabilities

In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities, which amends the accounting, presentation, and disclosure requirements for certain financial instruments. The ASU requires that all equity investments be recorded at fair value through net income (other than those accounted for under equity method or result in consolidation of the investee); however, an entity may choose to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. The ASU also requires an entity to present separately in other comprehensive income the portion of the total change in fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability under the fair value option. The ASU also clarifies that an entity must evaluate the need for a valuation allowance on a deferred tax asset related to available for sale securities in combination with the entity’s other deferred tax assets. In addition, the ASU amends the presentation and disclosure requirements for financial instruments and now requires the use of an exit price notion when measuring the fair value of financial instruments for disclosure purposes. The ASU is effective for interim and annual periods beginning on January 1, 2018, with early adoption permitted for the amendments to the accounting for financial liabilities under the fair value option. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Accounting for Leases

In February 2016, the FASB issued ASU 2016-02, Leases, which will require entities that lease assets (i.e., lessees) to recognize assets and liabilities on their balance sheet for the rights and obligations created by those leases. The accounting by entities that own the assets leased (i.e., lessors) will remain largely unchanged; however, leveraged lease accounting will no longer be permitted for leases that commence after the effective date. The ASU will also require qualitative and quantitative disclosures about the amount, timing and uncertainty of cash flows arising from leases. The ASU is effective for interim and annual periods beginning on January 1, 2019 and requires a modified retrospective approach, with early adoption permitted. The Company plans to adopt the ASU on January 1, 2019. The Company recently began the technology design phase of this project to support the ongoing lessee accounting required under the ASU. Management is currently assessing the impact of this guidance on the Company's financial position and results of operations.

Principal versus Agent Considerations (Reporting Revenue Gross versus Net)

In March 2016, the FASB issued ASU 2016-08, Principal versus Agent Considerations (Reporting Revenue Gross versus Net). The ASU amends ASU 2014-09, Revenue from Contracts with Customers, with respect to assessing whether an entity is a principal (and thus presents revenue gross) or an agent (and thus presents revenue net). The amendments retain the guidance that the principal in an arrangement controls a good or service before it is transferred to a customer and clarify: (1) that an entity must first identify the specified good or service being provided to the customer; (2) that the unit of account for the principal versus agent assessment is each specified good or service promised in a contract; (3) indicators and examples to help an entity evaluate whether it is the principal; and (4) how to assess whether an entity controls services performed by another party. The ASU is effective upon the adoption of ASU 2014-09, which the Company plans to adopt beginning January 1, 2018, with early adoption permitted in 2017. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Measurement of Credit Losses on Financial Instruments

In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments, which provides new guidance on the accounting for credit losses for instruments that are within its scope. For loans and debt securities accounted for at amortized cost, certain off-balance sheet credit exposures, net investments in leases, and trade receivables, the ASU requires an entity to recognize its estimate of credit losses expected over the life of the financial instrument or exposure. Lifetime expected credit losses on purchased financial assets with credit deterioration will be recognized as an allowance with an offset to the cost basis of the asset. For available for sale debt securities, the new standard will require recognition of expected credit losses by recognizing an allowance for credit losses when the fair value of the security is below amortized cost and the recognition of this allowance is limited to the difference between the security’s amortized cost basis and fair value. The ASU is effective for interim and annual periods beginning on January 1, 2020, with early adoption permitted in 2019. The Company plans to adopt the ASU on January 1, 2020. The Company is currently collecting business and data requirements to support the project planning phase of the implementation. Management is currently assessing the impact of this guidance on the Company’s financial position and results of operations.

Classification of Certain Cash Receipts and Cash Payments

In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments, to address diversity in practice in how certain cash receipts and payments are presented and classified in the statement of cash flows. The ASU is effective for interim and annual periods beginning on January 1, 2018, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company's statement of cash flows.

Income Tax Consequences of Intra-Entity Asset Transfers

In October 2016, the FASB issued ASU 2016-16, Intra-Entity Transfers of Assets Other Than Inventory, to improve the accounting for the income tax consequences of intra-entity assets other than inventory. The ASU will require recognition of the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The ASU is effective for interim and annual periods beginning on January 1, 2018, with early adoption permitted. Management does not expect adoption of this guidance to significantly impact the Company's financial position and results of operations.

Restricted Cash

In November 2016, the FASB issued ASU 2016-18, Restricted Cash, to address diversity in the classification and presentation of changes in restricted cash on the statement of cash flows. The ASU requires that a statement of cash flows explains the change during the period in the total of cash, cash equivalents, and the amounts generally described as restricted cash or restricted cash equivalents. The ASU is effective for interim and annual periods beginning on January 1, 2018 using a retrospective transition method. Early adoption is permitted. Management does not expect the adoption of this guidance to have a significant impact on the Company's statement of cash flows.

Clarifying the Definition of a Business

In January 2017, the FASB issued ASU 2017-01, Clarifying the Definition of a Business, which specifies when a set of assets and activities constitutes a business. The ASU adds a “screen” to determine when a set is not a business, thus reducing the number of transactions deemed businesses. Specifically, if the fair value of the gross assets acquired is concentrated in a single identifiable asset (or a group of similar identifiable assets), the set is not deemed a business. Otherwise, to be considered a business, a set must include at least one input and a substantive process that together significantly contribute to the ability to create outputs. Although outputs are not required to be a business, the ASU narrows the definition of an output and limits the instances where sets that lack outputs are deemed businesses. The ASU is effective for interim and annual periods prospectively beginning on January 1, 2018, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Simplifying the Test for Goodwill Impairment

In January 2017, the FASB issued ASU 2017-04, Simplifying the Test for Goodwill Impairment. The ASU removes Step 2 of the goodwill impairment test, which measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. Under the amendments, a goodwill impairment loss will be measured as the amount by which a reporting unit’s carrying amount exceeds its fair value; however, the loss recognized cannot exceed the total amount of goodwill allocated to that reporting unit. Additionally, an entity should consider income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit when measuring the goodwill impairment loss. The ASU will be effective for MUAH beginning January 1, 2020 on a prospective basis. Early adoption is permitted for any impairment tests performed after January 1, 2017. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position or results of operations.

Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of

Nonfinancial Assets

In February 2017, the FASB issued ASU 2017-05, Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets, to clarify the scope of ASC 610-20, Other Income - Gains and Losses from Derecognition of Nonfinancial Assets (issued as part of ASU 2014-09), and provide guidance on partial sales of nonfinancial assets. The ASU clarifies that the unit of account under ASC 610-20 is each distinct nonfinancial or in substance nonfinancial asset and that a financial asset that meets the definition of an “in substance nonfinancial asset” is within the scope of ASC 610-20. The ASU eliminates rules specifically addressing sales of real estate and removes exceptions to the financial asset derecognition model. The ASU is effective upon the adoption of ASU 2014-09, which the Company plans to adopt beginning January 1, 2018. It allows an entity to use either a retrospective or modified retrospective approach. Management does not expect the adoption of this guidance to significantly impact the Company's financial position and results of operations.

Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost

In March 2017, the FASB issued ASU 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, which amends the income statement presentation of the components of net periodic benefit cost for sponsored defined benefit pension and other postretirement plans. The ASU also now mandates that only the service cost component of net benefit cost is eligible for capitalization on certain internally produced assets. The ASU is effective for interim and annual periods beginning January 1, 2018, with retrospective application for the new income statement presentation requirements and prospective application for the new capitalization requirement. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Premium Amortization on Purchased Callable Debt Securities

In March 2017, the FASB issued ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities, which requires premiums on certain purchased callable debt securities to be amortized to the earliest call date. Under current guidance, premiums on callable debt securities are generally amortized over the contractual life of the security. The amortization period for callable debt securities purchased at a discount will not be impacted. The ASU is effective for interim and annual periods beginning on January 1, 2019 and requires a modified retrospective approach, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.

Scope of Modification Accounting

In May 2017, the FASB issued ASU 2017-09, Scope of Modification Accounting, to provide guidance about which changes to the terms and conditions of a share-based payment award require an entity to apply modification accounting. The ASU is effective for interim and annual periods beginning January 1, 2018, and will be applied prospectively, with early adoption permitted. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position and results of operations.