Shareholders' Equity - Key's and KeyBank's Actual Capital Amounts and Ratios, Minimum Capital Amounts and Ratios (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Key [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Net Risk-Weighted Assets, Actual Amount	$ 15,638	$ 11,671
Tier 1 Capital to Average Quarterly Tangible Assets, Amount	13,249	10,212
Tier 1 Capital to Net Risk-Weighted Assets, Actual Amount	$ 13,249	$ 10,212
Total Capital to Net Risk-Weighted Assets, Actual Ratio	12.85%	12.97%
Tier 1 Capital to Net Risk-Weighted Assets, Actual Ratio	10.89%	11.35%
Tier 1 Capital to Average Quarterly Tangible Assets, Actual Ratio	9.90%	10.72%
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	$ 9,734	$ 7,198
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	7,300	5,399
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Amount	$ 5,442	$ 3,811
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	8.00%	8.00%
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	6.00%	6.00%
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Ratio	4.00%	4.00%
KeyBank (consolidated) [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Net Risk-Weighted Assets, Actual Amount	$ 14,291	$ 10,454
Tier 1 Capital to Average Quarterly Tangible Assets, Amount	12,439	9,197
Tier 1 Capital to Net Risk-Weighted Assets, Actual Amount	$ 12,439	$ 9,197
Total Capital to Net Risk-Weighted Assets, Actual Ratio	12.36%	12.34%
Tier 1 Capital to Net Risk-Weighted Assets, Actual Ratio	10.76%	10.86%
Tier 1 Capital to Average Quarterly Tangible Assets, Actual Ratio	9.46%	9.89%
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	$ 9,251	$ 6,776
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Amount	6,939	5,082
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Amount	$ 5,353	$ 3,718
Total Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	8.00%	8.00%
Tier 1 Capital to Net Risk-Weighted Assets, to Meet Minimum Capital Adequacy Requirements, Ratio	6.00%	6.00%
Tier 1 Capital to Average Quarterly Tangible Assets to Meet Minimum Capital Adequacy Requirements, Ratio	4.00%	4.00%
Total Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Amount	$ 11,564	$ 8,470
Tier 1 Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Amount	6,939	5,082
Tier 1 Capital to Average Quarterly Tangible Assets to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Amount	$ 6,691	$ 4,647
Total Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Ratio	10.00%	10.00%
Tier 1 Capital to Net Risk-Weighted Assets, to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Ratio	6.00%	6.00%
Tier 1 Capital to Average Quarterly Tangible Assets to Qualify as Well Capitalized Under Federal Deposit Insurance Act, Ratio	5.00%	5.00%

Tangible Capital Qualify As Well Capitalized Under Federal Deposit Insurance Act

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Tangible Capital Qualify As Well Capitalized Under Federal Deposit Insurance Act Ratio

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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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.

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Tangible capital as defined in the regulations (generally includes outstanding stock plus retained earnings) for banking institutions subject to Office of Thrift Supervision (OTS) capital requirements.

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The minimum amount of tangible capital required under the regulatory framework for prompt corrective action.

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The minimum amount of Tangible Capital divided by tangible assets (Tangible Capital Ratio) as defined in the regulations.

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Tangible Capital divided by Tangible assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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