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Stock-Based Compensation - Narrative (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total compensation expense for stock-based compensation plans | $ 96,000,000 | $ 99,000,000 | $ 104,000,000
Total income tax benefit recognized for stock-based compensation plans | $ 23,000,000 | $ 23,000,000 | $ 39,000,000
Common Shares available for future grant under compensation plans (in shares) | 59,991,178 |  |
Maximum percentage of outstanding common stock that may be granted as options | 6.00% |  |
Rolling period in which a certain percentage of common stock cannot be granted as options | 3 years |  |
Exercise rate of stock options granted to employees | 25.00% |  |
Vesting period for compensation cost | 4 years |  |
Authorized number of shares that may be issued as equity awards (in shares) | 71,600,000 |  |
Common shares, shares issued (in shares) | 1,256,702,081 | 1,256,702,081 |
Weighted-average grant-date fair value of options (in dollars per share) | $ 3.07 | $ 5.12 | $ 4.60
Number of options, exercised (in shares) | 2,039,208 |  |
Total intrinsic value of exercised options | $ 18,000,000 | $ 21,000,000 | $ 31,000,000
Cash received from options exercised | 18,000,000 | 20,000,000 | 25,000,000
Actual tax benefit realized for tax deductions from options exercised | $ 1,000,000 | $ 1,000,000 | $ 4,000,000
Mandatory deferred incentive awards, vesting rate (as a percent) | 25.00% |  |
Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Common shares, shares issued (in shares) | 3,000,000 |  |
Stock Options [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Options expiration years | 10 years |  |
Number of options, exercised (in shares) | 2,039,208 | 1,960,444 | 3,755,177
Unrecognized compensation cost related to nonvested options expected to vest | $ 2,000,000 |  |
Weighted-average period | 2 years |  |
Stock Options [Member] | Minimum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period for compensation cost | 1 year |  |
Stock Options [Member] | Maximum [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Options expiration years | 10 years |  |
Deferred Cash Awards [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting percentage | 25.00% |  |
Restricted Stock Unit [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting percentage | 25.00% |  |
Equity Based Performance Stock Units [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period | 3 years |  |
Cash Performance Units [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period | 3 years |  |
Deferred Compensation Plans and Other Restricted Stock Award [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Weighted-average grant-date fair value granted (in dollars per share) | $ 17.57 | $ 20.77 | $ 18.55
Deferred Compensation Plans [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Unrecognized compensation cost related to nonvested options expected to vest | $ 17,000,000 |  |
Weighted-average period | 4 years |  |
Fair value of units/shares vested | $ 19,000,000 | $ 22,000,000 | $ 21,000,000
Long-Term Incentive Compensation Program [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Unrecognized compensation cost related to nonvested options expected to vest | $ 87,000,000 |  |
Weighted-average period | 2 years 3 months 18 days |  |
Fair value of units/shares vested | $ 89,000,000 | $ 93,000,000 | $ 76,000,000
Weighted-average grant-date fair value granted (in dollars per share) | $ 18.25 | $ 19.28 | $ 19.82
Discounted Stock Purchase Plan [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Employee discount on the purchase of stock through Discounted Stock Purchase Plan | 10.00% |  |
Purchases are limited to any month | $ 10,000 |  |
Purchases are limited to any calendar year | $ 50,000 |  |
Issuance of common shares (in shares) | 327,243 | 327,435 | 257,738
Weighted-average cost of common shares issued under the plan (in dollars per share) | 15.73 | 17.48 | 16.61
Vesting Contingent On Performance And Service Conditions, Payable In Stock [Member] | Long-Term Incentive Compensation Program [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of performance units vested (in shares) | 855,233 |  |
Weighted-average grant-date fair value granted (in dollars per share) | $ 17.51 |  |
Vesting Contingent On Performance And Service Conditions, Payable In Stock [Member] | Long-Term Incentive Compensation Program [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of performance units vested (in shares) |  | 508,799 |
Vesting Contingent On Performance And Service Conditions, Payable In Stock [Member] | Long-Term Incentive Compensation Program [Member] | Performance Units [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Fair value of units/shares vested | $ 9,000,000 | $ 7,000,000 |
Vesting Contingent On Performance And Service Conditions, Payable In Cash [Member] | Long-Term Incentive Compensation Program [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of performance units vested (in shares) | 1,139,582 |  |
Weighted-average grant-date fair value granted (in dollars per share) | $ 20.33 |  |
Vesting Contingent On Performance And Service Conditions, Payable In Cash [Member] | Long-Term Incentive Compensation Program [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of performance units vested (in shares) |  | 561,313 |
Vesting Contingent On Performance And Service Conditions, Payable In Cash [Member] | Long-Term Incentive Compensation Program [Member] | Performance Units [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Fair value of units/shares vested | $ 20,000,000 | $ 12,000,000 |

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Name: | us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis=key_VestingContingentOnPerformanceAndServiceConditionsPayableInCashMember
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Data Type: | na
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