Regulatory Capital Requirements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Capital Requirements
Tier 1 capital (to risk-weighted assets), ratio, To Be Well-Capitalized Under Prompt Corrective Action Provisions	8.00%	8.00%
MUFG Americas Holdings Corporation
Regulatory Capital Requirements
Common equity tier 1 capital (to risk-weighted assets), Actual	$ 14,256	$ 15,708
Tier 1 capital (to risk-weighted assets), Actual	14,256	15,708
Total capital (to risk-weighted assets), Actual	14,904	17,106
Tier 1 Leverage, Actual	$ 14,256	$ 15,708
Common equity tier 1 capital (to risk-weighted assets), ratio, Actual	13.96%	16.31%
Tier 1 capital (to risk-weighted assets), ratio, Actual	13.96%	16.31%
Total capital (to risk-weighted assets), ratio, Actual	14.60%	17.76%
Tier 1 leverage, ratio, Actual	8.77%	10.06%
Common Equity Tier One Risk Based Capital Required for Capital Adequacy	$ 6,508	$ 5,539
Tier 1 capital (to risk-weighted assets), Minimum Regulatory Requirement	8,039	6,984
Total capital (to risk-weighted assets), Minimum Regulatory Requirement	10,081	8,910
Tier 1 Leverage, Minimum Regulatory Requirement	$ 6,502	$ 6,245
Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	6.375%	5.75%
Tier 1 capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	7.875%	7.25%
Total capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	9.875%	9.25%
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	$ 13,316	$ 14,028
Tier 1 capital (to risk-weighted assets), Actual	13,316
Total capital (to risk-weighted assets), Actual	13,905	15,335
Tier 1 Leverage, Actual	$ 13,316	$ 14,028
Common equity tier 1 capital (to risk-weighted assets), ratio, Actual	14.45%	16.17%
Tier 1 capital (to risk-weighted assets), ratio, Actual	14.45%	16.17%
Total capital (to risk-weighted assets), ratio, Actual	15.09%	17.68%
Tier 1 leverage, ratio, Actual	10.61%	11.78%
Common Equity Tier One Risk Based Capital Required for Capital Adequacy	$ 5,876	$ 4,987
Tier 1 capital (to risk-weighted assets), Minimum Regulatory Requirement	7,258	6,288
Total capital (to risk-weighted assets), Minimum Regulatory Requirement	9,102	8,023
Tier 1 Leverage, Minimum Regulatory Requirement	$ 5,018	$ 4,762
Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	6.375%	5.75%
Tier 1 capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	7.875%	7.25%
Total capital (to risk-weighted assets), ratio, Minimum Regulatory Requirement	9.875%	9.25%
Tier 1 Leverage, ratio, Minimum Regulatory Requirement	4.00%	4.00%
Common Equity Tier One Risk Based Capital Required to be Well Capitalized	$ 5,991	$ 5,637
Tier 1 capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	7,374	6,938
Total capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	9,217	8,673
Tier 1 Leverage, To Be Well-Capitalized Under Prompt Corrective Action Provisions	$ 6,273	$ 5,953
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%