Shareholders' Equity - Key's and KeyBank's Actual Capital Amounts and Ratios, Minimum Capital Amounts and Ratios (Details) $ in Millions	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
KeyBank (consolidated)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total risk-based capital, actual amount	$ 20,518	$ 20,726
Common equity Tier 1 risk-based capital, actual amount	17,560	17,487
Tier 1 risk- based capital, actual amount	17,560	17,487
Tier 1 risk- based capital, leverage amount	$ 17,560	$ 17,487
Total risk-based capital, actual ratio	0.1512	0.1416
Common equity Tier 1 risk-based capital, actual ratio	0.1294	0.1215
Tier 1 risk-based capital, actual ratio	0.1294	0.1194
Tier 1 risk- based capital, leverage actual ratio	0.0942	0.0922
Total risk-based capital, regulatory minimum, ratio	0.0800	0.0800
Common equity Tier 1 risk-based capital, regulatory minimum, ratio	0.0450	0.0450
Tier 1 risk-based capital, regulatory minimum, ratio	0.0600	0.0600
Tier 1 risk-based capital, leverage regulatory minimum ratio	0.0400	0.0400
Total risk-based capital, regulatory minimum with stress capital buffer	0.1110	0.1050
Common equity Tier 1 risk-based capital, regulatory minimum with stress capital buffer	0.0760	0.0700
Tier 1 risk-based capital, regulatory minimum with stress capital buffer	0.0910	0.0850
Tier 1 risk based capital leverage, regulatory minimum with stress capital buffer	0.0400	0.0400
Total risk-based capital, capitalized	0.1000	0.1000
Common equity Tier 1 risk-based capital, capitalized	0.0650	0.0650
Tier 1 risk-based capital, capitalized	0.0800	0.0800
Tier 1 risk-based capital, leverage capitalized	0.0500	0.0500
Parent
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total risk-based capital, actual amount	$ 22,336	$ 21,028
Common equity Tier 1 risk-based capital, actual amount	16,489	14,894
Tier 1 risk- based capital, actual amount	18,934	17,340
Tier 1 risk- based capital, leverage amount	$ 18,934	$ 17,340
Total risk-based capital, actual ratio	0.1615	0.1415
Common equity Tier 1 risk-based capital, actual ratio	0.1192	0.1002
Tier 1 risk-based capital, actual ratio	0.1369	0.1167
Tier 1 risk- based capital, leverage actual ratio	0.1003	0.0903
Total risk-based capital, regulatory minimum, ratio	0.0800	0.0800
Common equity Tier 1 risk-based capital, regulatory minimum, ratio	0.0450	0.0450
Tier 1 risk-based capital, regulatory minimum, ratio	0.0600	0.0600
Tier 1 risk-based capital, leverage regulatory minimum ratio	0.0400	0.0400
Total risk-based capital, regulatory minimum with stress capital buffer	0.1110	0.1050
Common equity Tier 1 risk-based capital, regulatory minimum with stress capital buffer	0.0760	0.0700
Tier 1 risk-based capital, regulatory minimum with stress capital buffer	0.0910	0.0850
Tier 1 risk based capital leverage, regulatory minimum with stress capital buffer	0.0400	0.0400