Commercial Paper and Other Short-Term Borrowings (Details) ¥ in Billions	9 Months Ended
Sep. 30, 2017 USD ($)	Sep. 30, 2017 JPY (¥)	Dec. 31, 2016 USD ($)
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 6,026,000,000		$ 2,360,000,000
MUFG Union Bank N.A
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	5,118,000,000		$ 989,000,000
MUFG Union Bank N.A \| Federal Funds Purchased
Short-term Debt [Line Items]
Weighted average interest rate at period end			0.50%
Federal funds purchased	$ 0		$ 26,000,000
MUFG Union Bank N.A \| Commercial paper
Short-term Debt [Line Items]
Weighted average interest rate at period end	0.87%	0.87%	0.55%
Commercial paper	$ 118,000,000		$ 263,000,000
MUFG Union Bank N.A \| Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Weighted average interest rate at period end	1.10%	1.10%	0.59%
Federal home loan bank advances	$ 5,000,000,000		$ 700,000,000
MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Total commercial paper and other short-term borrowings	$ 908,000,000		$ 1,371,000,000
MUFG Americas Holding Corporation \| Affiliated Entity
Short-term Debt [Line Items]
Weighted average interest rate at period end	(0.06%)	(0.06%)	(0.04%)
Short term debt due to affiliates	$ 697,000,000	¥ 78	$ 692,000,000
Short-term debt due to BTMU, with weighted average interest rates of 1.68% and 0.49% at September 30, 2017 and December 31, 2016, respectively \| MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Weighted average interest rate at period end	1.68%	1.68%	0.49%
Short-term debt due to parent	$ 169,000,000		$ 679,000,000
Short-term debt due to BTMU, with a weighted average interest rate of 2.04% at September 30, 2017 \| MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Weighted average interest rate at period end	2.04%	2.04%
Short-term debt due to parent	$ 42,000,000		$ 0
Line of Credit \| MUFG Americas Holding Corporation
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 1,400,000,000.0
Line of credit facility, extension period (in days)	100 days

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