Long-Term Debt (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 15,150		$ 17,129
Proceeds from issuance of long-term debt	624	$ 2,331
Parent Company [Member]
Debt Instrument [Line Items]
Long-term debt	7,625		7,622
Parent Company [Member] | Senior debt obligations | Fixed rate 3.50% notes due June 2022
Debt Instrument [Line Items]
Long-term debt	$ 399		$ 399
Debt instrument, fixed interest rate (as a percent)	3.50%		3.50%
Parent Company [Member] | Senior debt obligations | Fixed rate 3.00% notes due February 2025
Debt Instrument [Line Items]
Long-term debt	$ 398		$ 397
Debt instrument, fixed interest rate (as a percent)	3.00%		3.00%
Parent Company [Member] | Senior debt obligations | Floating rate debt due December 2023. This note, which bears interest at 0.76% above 3-month EURIBOR, had a rate of 0.76% at September 30, 2020 and 0.76% at December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 26		$ 24
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022
Debt Instrument [Line Items]
Long-term debt	$ 3,250		$ 3,250
Long-term debt, weighted average interest rate (as a percent)	1.14%		3.03%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 | London Interbank Offered Rate LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.90%		0.90%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 issued in Q3 2019
Debt Instrument [Line Items]
Long-term debt	$ 125		$ 125
Long-term debt, weighted average interest rate (as a percent)	1.21%		3.07%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 issued in Q3 2019 | London Interbank Offered Rate LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.97%		0.97%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR
Debt Instrument [Line Items]
Long-term debt	$ 1,625		$ 1,625
Long-term debt, weighted average interest rate (as a percent)	1.23%		3.12%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR | London Interbank Offered Rate LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.99%		0.99%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR Issued in Q42019 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,765
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR Issued Q4 2019
Debt Instrument [Line Items]
Long-term debt			$ 1,765
Long-term debt, weighted average interest rate (as a percent)	1.18%		2.82%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR Issued Q4 2019 | London Interbank Offered Rate LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.94%		0.94%
Parent Company [Member] | Subordinated Debt | Floating rate subordinated debt due December 2023. This note, which bears interest at 1.38% above 3-month LIBOR, had a rate of 3.71% at September 30, 2018 and 3.07% at December 31, 2017
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	0.76%		0.76%
Parent Company [Member] | Subordinated Debt | Floating rate subordinated debt due December 2023. This note, which bears interest at 1.38% above 3-month LIBOR, had a rate of 3.71% at September 30, 2018 and 3.07% at December 31, 2017 | Euro Interbank Offered Rate (Euribor)
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.76%		0.76%
Parent Company [Member] | Junior subordinated debt payable to trusts | Floating rate note due September 2036. This note had an interest rate of 1.95% at September 30, 2020 and 3.59% at December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 37		$ 37
Long-term debt, weighted average interest rate (as a percent)	1.95%		3.59%
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	$ 6,984
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022
Debt Instrument [Line Items]
Long-term debt	300		$ 300
MUFG Union Bank N.A | Senior debt obligations | Variable Rate FHLB advances due October 2020
Debt Instrument [Line Items]
Long-term debt	100		1,100
MUFG Union Bank N.A | Senior debt obligations | Floating Rate Debt Due December 2022
Debt Instrument [Line Items]
Long-term debt	299		300
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	6,984		8,909
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate 2.10 Percent Notes due December 2022
Debt Instrument [Line Items]
Long-term debt	$ 699		$ 698
Debt instrument, fixed interest rate (as a percent)	2.10%		2.10%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.85%		2.49%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 | London Interbank Offered Rate LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.60%		0.60%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.71%		0.71%
MUFG Union Bank N.A | Senior debt obligations | Variable Rate FHLB advances due October 2020
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.28%		1.90%
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate 3.15 Percent Notes Due April 2022
Debt Instrument [Line Items]
Long-term debt	$ 998		$ 998
Debt instrument, fixed interest rate (as a percent)	3.15%		3.15%
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate Advances Due Between October 2019 and December 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 4,575		$ 5,500
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate Advances Due Between July 2019 and December 2023
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	2.76%		2.95%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate Debt Due December 2022
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.79%		2.26%
MUFG Union Bank N.A | Other
Debt Instrument [Line Items]
Long-term debt	$ 13		$ 13
Other Subsidiaries, Excluding MUFG Bank, NA
Debt Instrument [Line Items]
Long-term debt	541		598
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Floating Rate Borrowings Due Between December 2020 and May 2021
Debt Instrument [Line Items]
Long-term debt	$ 0		$ 250
Long-term debt, weighted average interest rate (as a percent)	0.00%		1.94%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between January 2020 and June 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 333		$ 137
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between September 2019 and May 2024
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	1.50%		2.22%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between September 2019 and May 2024 | Minimum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	1.23%		1.68%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between September 2019 and May 2024 | Maximum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	2.44%		2.44%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due December 2023. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 2.25% at September 30, 2020 and 3.93% at December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 36		$ 13
Long-term debt, weighted average interest rate (as a percent)	2.25%		3.93%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed rate non-recourse borrowings due December 2026 which had an interest rate of 5.34% at September 30, 2020 and 5.34% December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 27		$ 29
Debt instrument, fixed interest rate (as a percent)	5.34%		5.34%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt December 2021
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	0.66%		2.17%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between December 2019 and July 2023
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	3.02%		3.07%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between December 2019 and July 2023 | Minimum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	1.96%		1.96%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between December 2019 and July 2023 | Maximum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	3.72%		3.72%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between February 2019 and December 2021
Debt Instrument [Line Items]
Long-term debt	$ 3		$ 3
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed Rate Nonrecourse Debt Due Between March 2020 and July 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 142		$ 166