Basis of Presentation and Accounting Policies Basis of Presentation and Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Corporation adopted the provision of Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 606, "Revenue from Contracts with Customers" (Topic 606), using the modified retrospective method applied to all open contracts as of January 1, 2018. Under Topic 606, card fee revenue from certain products is generally presented net of network costs, including interchange costs, surcharge fees and assessment fees, as opposed to the previous presentation of associated network costs in outside processing fees in the Consolidated Statements of Comprehensive Income. Similar adjustments were made for other revenue streams that resulted in certain costs being recognized in the same category as the associated revenues in noninterest income. The adoption of Topic 606 resulted in decreases of $34 million in card fees and $2 million in service charges on deposits accounts, included in noninterest income, and a corresponding $36 million decrease in outside processing fees included in noninterest expenses, in the Consolidated Statements of Comprehensive Income for the three months ended September 30, 2018. For the nine months ended September 30, 2018, the impact on the Consolidated Statements of Comprehensive Income was a $105 million decrease in card fees, a $4 million decrease in service charges on deposit accounts and a $109 million decrease in outside processing fee expense. The Corporation previously deferred recognition of certain treasury management fees included in service charges on deposit accounts in the Consolidated Statements of Comprehensive Income until the amount of compensation was considered fixed and determinable. Under the new guidance, the portion of these fees that are based on agreed-upon rates less estimated credits expected to be earned by the customer is recognized as services are rendered. As a result, the Corporation recorded a transition adjustment of $14 million, after tax, to retained earnings, included in cumulative effect of change in accounting principles in the accompanying Consolidated Statements of Changes in Shareholders Equity. Similar adjustments were made for other revenue streams that resulted in an additional cumulative transition after-tax adjustment to retained earnings of $2 million. Revenues from contracts with customers may be recognized when services are complete or as they are rendered, although contracts are generally short-term by nature. Services provided over a period of time are typically transferred to customers evenly over the term of the contracts and revenue is recognized evenly over the period services are provided. Contract receivables are included in accrued income and other assets on the Consolidated Balance Sheets. Payment terms vary by services offered, and the time between completion of performance obligations and payment is typically not significant. Card fees comprise interchange and other fee income earned on government card, commercial card, debit/automated teller machine card and merchant payment processing programs. Card fees are presented net of network costs, as performance obligations for card services are limited to transaction processing and settlement with the card network on behalf of the customers. Network costs were approximately $34 million and $29 million for the three months ended September 30, 2018 and 2017, respectively, and $105 million and $80 million for the nine months ended September 30, 2018 and 2017, respectively. Fees for these services are primarily based on interchange rates set by the network and transaction volume. The Corporation also provides ongoing card program support services, for which fees are based on contractually agreed prices and customer demand for services. Service charges on deposit accounts comprise charges on retail and business accounts, including fees for treasury management services. These treasury management services include transaction-based services related to payment processing, overdrafts, non-sufficient funds and other deposit account activity, as well as account management services that are provided over time. Business customers can earn credits depending on deposit balances maintained with the Corporation, which may be used to offset fees. Fees and credits are based on predetermined, agreed-upon rates. Fiduciary income includes fees and commissions from asset management, custody, recordkeeping, investment advisory and other services provided primarily to personal and institutional trust customers. Revenue is recognized as the services are performed and is based either on the market value of the assets managed or the services provided, as well as agreed-upon rates. Commercial lending fees include both revenue from contracts with customers (primarily loan servicing fees) and other sources of revenue. Commercial loan servicing fees are based on contractually agreed-upon prices and when the services are provided. Other sources of revenue in commercial lending fees primarily include fees assessed on the unused portion of commercial lines of credit (unused commitment fees) and syndication arrangements. Brokerage fees are commissions earned for facilitating securities transactions for customers, as well as other brokerage services provided. Revenue is recognized when services are complete and are based on the type of services provided and agreed-upon rates. The Corporation pays commissions based on brokerage fee revenue. These are typically recognized when incurred because the amortization period is one year or less and are included in salaries and benefits expense in the Consolidated Statements of Comprehensive Income. Other revenues, consisting primarily of other retail fees, investment banking fees and insurance commissions, are typically recognized when services or transactions are completed and are based on the type of services provided and agreed-upon rates. Except as discussed above, commissions and other incentives paid to employees are generally based on several internal and external metrics and, as a result, are not solely dependent on revenue generating activities.
Classification and Measurement of Financial Instruments	Classification and Measurement of Financial Instruments Effective January 1, 2018, the Corporation adopted the provisions of Accounting Standards Update (ASU) No. 2016-01, "Financial Instruments - Overall (Subtopic 825-10): Recognition of Financial Assets and Financial Liabilities," (ASU 2016-01). ASU 2016-01 requires equity investments, other than equity method investments, to be measured at fair value with changes in fair value recognized in net income. At adoption, an immaterial amount of cumulative net unrealized losses on equity securities previously recognized in accumulated other comprehensive income (AOCI) was reclassified to the opening balance of retained earnings, included in cumulative effect of change in accounting principles in the accompanying Consolidated Statements of Changes in Shareholders Equity. Changes to the fair value of equity securities occurring after December 31, 2017, other than equity method investments, are included in net securities losses in the Consolidated Statements of Comprehensive Income. Also, as part of adopting ASU 2016-01, the Corporation refined the calculation used to determine the estimated fair value of loans disclosed in note 2 to the consolidated financial statements.
Statement of Cash Flows	Statement of Cash Flows The Corporation adopted FASB issued ASU No. 2016-15, “Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments,” (ASU 2016-15) on January 1, 2018 and, as a result, reclassified $8 million of proceeds from settlement of bank-owned life insurance policies from operating activities to investing activities for the nine-month period ended September 30, 2017.
Defined Benefit Pension and Other Postretirement Costs	Defined Benefit Pension and Other Postretirement Costs The Corporation retrospectively adopted the provisions of ASU No. 2017-07, “Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost” (ASU 2017-07) on January 1, 2018, which requires employers to report service cost as part of compensation expense and the other components of net benefit credit separately from service cost. As a result, $12 million and $36 million of benefit from the other components of net benefit credit was reclassified from salaries and benefits expense to other noninterest expenses in the Consolidated Statements of Comprehensive Income for the three and nine months ended September 30, 2017, respectively. The Corporation based the adjustment to the prior periods on amounts disclosed in note 10.
Derivatives Instruments and Hedging Activities	Derivatives Instruments and Hedging Activities The Corporation adopted ASU No. 2017-12, “Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities” (ASU 2017-12), effective January 1, 2018. At transition, the Corporation elected to change the measurement methodology of all long-haul fair value hedges existing at December 31, 2017. The prior period effect of this election was a $1 million reduction to opening retained earnings, included in cumulative effect of change in accounting principles in the Consolidated Statements of Shareholders' Equity. In addition, the Corporation made a transition election to reclassify the portfolio of held-to-maturity securities to available-for-sale in January 2018 as the securities are eligible to be hedged. This resulted in the recognition of additional unrealized losses of $11 million at the date of transfer.
Income Taxes	Income Taxes The Tax Cuts and Jobs Act (the "Act"), enacted on December 22, 2017, reduced the U.S. federal corporate tax rate from 35 percent to 21 percent. Also, on December 22, 2017, the Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 (SAB 118), which provides guidance on accounting for tax effects of the Act. SAB 118 provides a measurement period of up to one year from the enactment date to complete the accounting. Based on the information available and current interpretation of the rules, the Corporation has made reasonable estimates of the impact of the reduction in the corporate tax rate and remeasurement of certain deferred tax assets and liabilities based on the rate at which they are expected to reverse in the future, generally 21 percent. The provisional amount recorded related to the remeasurement of the Corporation's deferred tax balance was $99 million, including $107 million recognized in the year ended December 31, 2017 and an $8 million downward revision to the estimated impact recorded in the nine months ended September 30, 2018. The final impact of the Act may differ from these estimates as a result of changes in management’s interpretations and assumptions, as well as new guidance that may be issued by the Internal Revenue Service (IRS).
Pending Accounting Pronouncements	Pending Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842),” (ASU 2016-02), to increase the transparency and comparability of lease recognition and disclosure. ASU 2016-02 requires lessees to recognize lease contracts with a term greater than one year on the balance sheet, while recognizing expenses on the income statement in a manner similar to current guidance. ASU 2016-02 is effective for the Corporation on January 1, 2019 and must be applied using the modified retrospective approach. In July 2018, the FASB issued ASU 2018-11 “Leases (Topic 842): Targeted Improvements,” which provides lessees the option to apply the new leasing standard to all open leases as of the adoption date. The Corporation expects to make use of this transition option and is currently finalizing review of key assumptions and evaluation of service contracts for embedded leases. Based on preliminary evaluation, the right-of-use asset and corresponding lease obligation liability are expected to range between $450 million and $550 million at adoption, resulting in an 8- to 10-basis point decrease in the common equity tier 1 capital (CET1) ratio. Preliminary estimates are based on the current interest rate environment which may differ from those at the time of adoption of the standard. The Corporation will continue to evaluate other impacts of adoption but does not anticipate these to be significant. In June 2016, the FASB issued ASU No. 2016-13, "Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments," (ASU 2016-13), which addresses concerns regarding the perceived delay in recognition of credit losses under the existing incurred loss model. The amendment introduces a new, single model for recognizing credit losses on all financial instruments presented on a cost basis. Under the new model, entities must estimate current expected credit losses by considering all available relevant information, including historical and current information, as well as reasonable and supportable forecasts of future events. The update also requires additional qualitative and quantitative information to allow users to better understand the credit risk within the portfolio and the methodologies for determining the allowance for credit losses. ASU 2016-13 is effective for the Corporation on January 1, 2020 and must be applied using the modified retrospective approach with limited exceptions. Early adoption is permitted. The Corporation will adopt the standard on January 1, 2020 and is currently evaluating the impact of adoption. In August 2018, the FASB issued ASU No. 2018-15, "Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract," (ASU 2018-15), to align the requirements for capitalizing implementation costs in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs relating to internal-use software. The ASU 2018-15 update requires entities in a hosting arrangement that is a service contract to follow the guidance in Subtopic 350-40 to determine which implementation costs to capitalize as an asset and which costs to expense. ASU 2018-15 is effective for the Corporation on January 1, 2020 and may be applied using either the retrospective or prospective approach. Early adoption is permitted. The Corporation is currently evaluating the impact of adopting ASU 2018-15.

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

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Disclosure of accounting policy for pension and other postretirement benefit plans. This accounting policy may address (1) the types of plans sponsored by the entity, and the benefits provided by each plan (2) groups that participate in (or are covered by) each plan (3) how plan assets, liabilities and expenses are measured, including the use of any actuaries and (4) significant assumptions used by the entity to value plan assets and liabilities and how such assumptions are derived.

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