Regulatory Capital Requirements (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
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	$ 15,086	$ 14,256
Tier 1 capital (to risk-weighted assets), Actual	15,086	14,256
Total capital (to risk-weighted assets), Actual	15,769	14,904
Tier 1 Leverage, Actual	$ 15,086	$ 14,256
Common equity tier 1 capital (to risk-weighted assets), ratio, Actual	14.10%	13.96%
Tier 1 capital (to risk-weighted assets), ratio, Actual	14.10%	13.96%
Total capital (to risk-weighted assets), ratio, Actual	14.73%	14.60%
Tier 1 leverage, ratio, Actual	8.88%	8.77%
Common Equity Tier One Risk Based Capital Required for Capital Adequacy	$ 7,492	$ 6,508
Tier 1 capital (to risk-weighted assets), Minimum Regulatory Requirement	9,097	8,039
Total capital (to risk-weighted assets), Minimum Regulatory Requirement	11,237	10,081
Tier 1 Leverage, Minimum Regulatory Requirement	$ 6,792	$ 6,502
Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	7.00%	6.375%
Tier 1 capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	8.50%	7.875%
Total capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	10.50%	9.875%
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	$ 14,115	$ 13,316
Tier 1 capital (to risk-weighted assets), Actual	14,115
Total capital (to risk-weighted assets), Actual	14,746	13,905
Tier 1 Leverage, Actual	$ 14,115	$ 13,316
Common equity tier 1 capital (to risk-weighted assets), ratio, Actual	14.47%	14.45%
Tier 1 capital (to risk-weighted assets), ratio, Actual	14.47%	14.45%
Total capital (to risk-weighted assets), ratio, Actual	15.11%	15.09%
Tier 1 leverage, ratio, Actual	10.65%	10.61%
Common Equity Tier One Risk Based Capital Required for Capital Adequacy	$ 6,829	$ 5,876
Tier 1 capital (to risk-weighted assets), Minimum Regulatory Requirement	8,293	7,258
Total capital (to risk-weighted assets), Minimum Regulatory Requirement	10,244	9,102
Tier 1 Leverage, Minimum Regulatory Requirement	$ 5,304	$ 5,018
Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	7.00%	6.375%
Tier 1 capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	8.50%	7.875%
Total capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	10.50%	9.875%
Tier 1 Leverage, ratio, Minimum Regulatory Requirement	4.00%	4.00%
Common Equity Tier One Risk Based Capital Required to be Well Capitalized	$ 6,342	$ 5,991
Tier 1 capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	7,805	7,374
Total capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	9,756	9,217
Tier 1 Leverage, To Be Well-Capitalized Under Prompt Corrective Action Provisions	$ 6,629	$ 6,273
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%