Commercial Paper and Other Short-Term Borrowings (Details) $ in Millions, ¥ in Billions	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 JPY (¥)	Dec. 31, 2017 USD ($)
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 9,764		$ 7,066
MUFG Union Bank N.A
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	8,662		$ 6,097
MUFG Union Bank N.A | Commercial paper
Short-term Debt [Line Items]
Weighted average interest rate at period end			1.26%
Commercial paper	$ 462		$ 347
MUFG Union Bank N.A | Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Weighted average interest rate at period end	2.06%	2.06%	1.42%
Federal home loan bank advances	$ 8,200		$ 5,750
MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Weighted average interest rate at period end	2.64%	2.64%	1.88%
Total commercial paper and other short-term borrowings	$ 1,102		$ 969
Short-term debt due to parent	$ 151		$ 168
MUFG Americas Holding Corporation | Affiliated Entity
Short-term Debt [Line Items]
Weighted average interest rate at period end	(0.08%)	(0.08%)	(0.09%)
Short term debt due to affiliates	$ 951	¥ 105	$ 801
Line of Credit | MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 1,400	¥ 160
Line of credit facility, extension period (in days)	100 days