Summary of Significant Accounting Policies and Nature of Operations	3 Months Ended
Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies and Nature of Operations	Summary of Significant Accounting Policies and Nature of Operations MUFG Americas Holdings Corporation (MUAH) is a financial holding company, bank holding company and intermediate holding company whose principal subsidiaries are MUFG Union Bank, N.A. (MUB or the Bank) and MUFG Securities Americas Inc. (MUSA). MUAH provides a wide range of financial services to consumers, small businesses, middle-market companies and major corporations nationally and internationally. The Company also provides various business, banking, financial, administrative and support services, and facilities for MUFG Bank, Ltd. (formerly The Bank of Tokyo-Mitsubishi UFJ, Ltd.) in connection with the operation and administration of MUFG Bank, Ltd.'s business in the U.S. (including MUFG Bank, Ltd.'s U.S. branches). All of the Company's issued and outstanding shares of common stock are owned by MUFG Bank, Ltd. and MUFG. 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. The results of operations for the first quarter of 2019 are not necessarily indicative of the operating results anticipated for the full year. These unaudited Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 (2018 Form 10-K). 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 "Loans and Allowance for Loan Losses"), goodwill impairment, fair value of financial instruments (Note 8 "Fair Value Measurement and Fair Value of Financial Instruments"), pension accounting (Note 11 "Employee Pension and Other Postretirement Benefits"), income taxes, and transfer pricing. Recently Issued Accounting Pronouncements 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. This new guidance is commonly referred to as the current expected credit loss (“CECL”) model. 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 has completed the design phase of the implementation and will begin parallel testing in the second quarter of 2019. Management currently expects the allowance for credit losses will increase upon adoption of this ASU; however, the magnitude of the change is highly dependent on economic conditions and portfolio composition at the time of adoption. 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. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position or results of operations. Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, which modifies disclosure requirements on fair value measurements to improve their effectiveness. The guidance permits entities to consider materiality when evaluating fair value measurement disclosures and, among other modifications, requires certain new disclosures related to Level 3 fair value measurements. The guidance will be effective for MUAH beginning January 1, 2020, with early adoption permitted. The guidance only affects disclosures in the notes to the consolidated financial statements and will not affect the Company’s financial position or results of operations. Recently Adopted Accounting Pronouncements Accounting for Leases In February 2016, the FASB issued ASU 2016-02, Leases, which requires 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) remains largely unchanged; however, leveraged lease accounting is no longer permitted for leases that commence after the effective date. The ASU also requires 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. In July 2018, the FASB issued ASU 2018-11, Leases, which allows entities the option to recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. In December 2018, the FASB issued ASU 2018-20, Leases, which clarified lessor accounting for sales and similar taxes collected by lessors, certain lessor costs, and lease payments with lease and nonlease components. Effective January 1, 2019, the Company adopted this guidance which did not significantly impact the Company's financial position or 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. Effective January 1, 2019, the Company adopted this guidance which did not significantly impact the Company's financial position or results of operations. Targeted Improvements to Accounting for Hedging Activities In August 2017, the FASB issued ASU 2017-12, Targeted Improvements to Accounting for Hedging Activities, which will make more hedging strategies eligible for hedge accounting, simplify the application of hedge accounting, and enhance the transparency and understandability of hedge results. The guidance eliminates the requirement to separately measure and report hedge ineffectiveness and generally requires the entire change in the fair value of a hedging instrument to be presented in the same income statement line as the hedged item. It also amends the disclosure requirements and changes how entities assess effectiveness. Effective January 1, 2019, the Company adopted this guidance which did not significantly impact the Company's financial position or results of operations. As permitted by this guidance, the Company transferred securities held to maturity with a carrying amount of $170 million to securities available for sale upon adoption.

Summary of Significant Accounting Policies and Nature of Operations

MUFG Americas Holdings Corporation (MUAH) is a financial holding company, bank holding company and intermediate holding company whose principal subsidiaries are MUFG Union Bank, N.A. (MUB or the Bank) and MUFG Securities Americas Inc. (MUSA). MUAH provides a wide range of financial services to consumers, small businesses, middle-market companies and major corporations nationally and internationally. The Company also provides various business, banking, financial, administrative and support services, and facilities for MUFG Bank, Ltd. (formerly The Bank of Tokyo-Mitsubishi UFJ, Ltd.) in connection with the operation and administration of MUFG Bank, Ltd.'s business in the U.S. (including MUFG Bank, Ltd.'s U.S. branches). All of the Company's issued and outstanding shares of common stock are owned by MUFG Bank, Ltd. and MUFG. 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. The results of operations for the first quarter of 2019 are not necessarily indicative of the operating results anticipated for the full year. These unaudited Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 (2018 Form 10-K).

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 "Loans and Allowance for Loan Losses"), goodwill impairment, fair value of financial instruments (Note 8 "Fair Value Measurement and Fair Value of Financial Instruments"), pension accounting (Note 11 "Employee Pension and Other Postretirement Benefits"), income taxes, and transfer pricing.

Recently Issued Accounting Pronouncements

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. This new guidance is commonly referred to as the current expected credit loss (“CECL”) model. 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 has completed the design phase of the implementation and will begin parallel testing in the second quarter of 2019. Management currently expects the allowance for credit losses will increase upon adoption of this ASU; however, the magnitude of the change is highly dependent on economic conditions and portfolio composition at the time of adoption.

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. Management does not expect the adoption of this guidance to significantly impact the Company’s financial position or results of operations.

Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement

In August 2018, the FASB issued ASU 2018-13, Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement, which modifies disclosure requirements on fair value measurements to improve their effectiveness. The guidance permits entities to consider materiality when evaluating fair value measurement disclosures and, among other modifications, requires certain new disclosures related to Level 3 fair value measurements. The guidance will be effective for MUAH beginning January 1, 2020, with early adoption permitted. The guidance only affects disclosures in the notes to the consolidated financial statements and will not affect the Company’s financial position or results of operations.

Recently Adopted Accounting Pronouncements

Accounting for Leases

In February 2016, the FASB issued ASU 2016-02, Leases, which requires 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) remains largely unchanged; however, leveraged lease accounting is no longer permitted for leases that commence after the effective date. The ASU also requires 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. In July 2018, the FASB issued ASU 2018-11, Leases, which allows entities the option to recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. In December 2018, the FASB issued ASU 2018-20, Leases, which clarified lessor accounting for sales and similar taxes collected by lessors, certain lessor costs, and lease payments with lease and nonlease components. Effective January 1, 2019, the Company adopted this guidance which did not significantly impact the Company's financial position or 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. Effective January 1, 2019, the Company adopted this guidance which did not significantly impact the Company's financial position or results of operations.

Targeted Improvements to Accounting for Hedging Activities

In August 2017, the FASB issued ASU 2017-12, Targeted Improvements to Accounting for Hedging Activities, which will make more hedging strategies eligible for hedge accounting, simplify the application of hedge accounting, and enhance the transparency and understandability of hedge results. The guidance eliminates the requirement to separately measure and report hedge ineffectiveness and generally requires the entire change in the fair value of a hedging instrument to be presented in the same income statement line as the hedged item. It also amends the disclosure requirements and changes how entities assess effectiveness. Effective January 1, 2019, the Company adopted this guidance which did not significantly impact the Company's financial position or results of operations. As permitted by this guidance, the Company transferred securities held to maturity with a carrying amount of $170 million to securities available for sale upon adoption.