Regulatory Capital Requirements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Regulatory Capital Requirements
Tier 1 capital (to risk-weighted assets), ratio, To Be Well-Capitalized Under Prompt Corrective Action Provisions	8.00%	8.00%
Total capital (to risk-weighted assets), ratio, To Be Well-Capitalized Under Prompt Corrective Action Provisions	10.00%
MUFG Americas Holdings Corporation
Regulatory Capital Requirements
Common equity tier 1 capital (to risk-weighted assets), Actual	$ 15,823	$ 15,086
Tier 1 capital (to risk-weighted assets), Actual	15,823	15,086
Total capital (to risk-weighted assets), Actual	16,871	15,769
Tier 1 Leverage, Actual	$ 15,823	$ 15,086
Common equity tier 1 capital (to risk-weighted assets), ratio, Actual	15.28%	14.10%
Tier 1 capital (to risk-weighted assets), ratio, Actual	15.28%	14.10%
Total capital (to risk-weighted assets), ratio, Actual	16.29%	14.73%
Tier 1 leverage, ratio, Actual	9.56%	8.88%
Common Equity Tier One Risk Based Capital Required for Capital Adequacy	$ 9,218	$ 7,492
Tier 1 capital (to risk-weighted assets), Minimum Regulatory Requirement	10,772	9,097
Total capital (to risk-weighted assets), Minimum Regulatory Requirement	12,843	11,237
Tier 1 Leverage, Minimum Regulatory Requirement	$ 6,623	$ 6,792
Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	8.90%	7.00%
Tier 1 capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	10.40%	8.50%
Total capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	12.40%	10.50%
Tier 1 Leverage, ratio, Minimum Regulatory Requirement	4.00%	4.00%
Bank
Regulatory Capital Requirements
Common equity tier 1 capital (to risk-weighted assets), Actual	$ 14,634	$ 14,115
Tier 1 capital (to risk-weighted assets), Actual	14,634
Total capital (to risk-weighted assets), Actual	15,629	14,746
Tier 1 Leverage, Actual	$ 14,634	$ 14,115
Common equity tier 1 capital (to risk-weighted assets), ratio, Actual	15.62%	14.47%
Tier 1 capital (to risk-weighted assets), ratio, Actual	15.62%	14.47%
Total capital (to risk-weighted assets), ratio, Actual	16.68%	15.11%
Tier 1 leverage, ratio, Actual	11.12%	10.65%
Common Equity Tier One Risk Based Capital Required for Capital Adequacy	$ 6,557	$ 6,829
Tier 1 capital (to risk-weighted assets), Minimum Regulatory Requirement	7,963	8,293
Total capital (to risk-weighted assets), Minimum Regulatory Requirement	9,836	10,244
Tier 1 Leverage, Minimum Regulatory Requirement	$ 5,262	$ 5,304
Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	7.00%	7.00%
Tier 1 capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	8.50%	8.50%
Total capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	10.50%	10.50%
Tier 1 Leverage, ratio, Minimum Regulatory Requirement	4.00%	4.00%
Common Equity Tier One Risk Based Capital Required to be Well Capitalized	$ 6,089	$ 6,342
Tier 1 capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	7,494	7,805
Total capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	9,368	9,756
Tier 1 Leverage, To Be Well-Capitalized Under Prompt Corrective Action Provisions	$ 6,577	$ 6,629
Common Equity Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Total capital (to risk-weighted assets), ratio, To Be Well-Capitalized Under Prompt Corrective Action Provisions	10.00%	10.00%
Tier 1 Leverage, ratio, To Be Well-Capitalized Under Prompt Corrective Action Provisions	5.00%	5.00%