Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Details) $ in Millions, ¥ in Billions	Dec. 31, 2019 USD ($)	Dec. 31, 2019 JPY (¥)	Dec. 31, 2018 USD ($)
Commercial Paper and Short-Term Borrowings
Total commercial paper and other short-term borrowings	$ 6,484		$ 9,263
MUFG Union Bank N.A [Member]
Commercial Paper and Short-Term Borrowings
Total commercial paper and other short-term borrowings	6,158		8,182
MUAH
Commercial Paper and Short-Term Borrowings
Short-term debt due to MUFG Bank, Ltd., with weighted average interest rates of 2.27% and 3.01% at December 31, 2019 and December 31, 2018, respectively	69		145
Total commercial paper and other short-term borrowings	326		$ 1,081
Weighted average interest rate			3.01%
Commercial paper | MUFG Union Bank N.A [Member]
Commercial Paper and Short-Term Borrowings
Commercial paper, with a weighted average interest rate of 1.98% and 2.39% at December 31, 2019 and December 31, 2018, respectively	$ 58		$ 382
Weighted average interest rate	1.98%	1.98%	2.39%
Federal Home Loan Bank advances | MUFG Union Bank N.A [Member]
Commercial Paper and Short-Term Borrowings
Federal Home Loan Bank advances, with a weighted average interest rate of 1.86% and 2.52% at December 31, 2019 and December 31, 2018, respectively	$ 6,100		$ 7,800
Weighted average interest rate	1.86%	1.86%	2.52%
Parent Company [Member] | MUAH
Commercial Paper and Short-Term Borrowings
Weighted average interest rate	2.27%	2.27%
Affiliated Entity | MUAH
Commercial Paper and Short-Term Borrowings
Short-term debt due to affiliates, with weighted average interest rates of (-0.10)% and (-0.07)% at December 31, 2019 and December 31, 2018, respectively	$ 257	¥ 28	$ 936
Weighted average interest rate	(0.10%)	(0.10%)	(0.07%)