Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 16,686			$ 17,129
Proceeds from issuance of long-term debt	505	$ 1,300
Parent Company [Member]
Debt Instrument [Line Items]
Long-term debt	7,623			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 March 31, 2020 and 0.76% at December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 24			$ 24
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 3,250			$ 3,250
Long-term debt, weighted average interest rate (as a percent)	1.68%			3.03%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 [Member] | 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 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 125			$ 125
Long-term debt, weighted average interest rate (as a percent)	1.75%			3.07%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 issued in Q3 2019 [Member] | 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 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,625			$ 1,625
Long-term debt, weighted average interest rate (as a percent)	1.77%			3.12%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR [Member] | 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 Q4 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,765			$ 1,765
Long-term debt, weighted average interest rate (as a percent)	1.72%			2.82%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR Issued Q4 2019 [Member] | 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) [Member]
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 2.44% at March 31, 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)	2.44%			3.59%
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	$ 8,508
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	300			$ 300
MUFG Union Bank N.A | Senior debt obligations | Variable Rate FHLB advances due October 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	1,100			1,100
MUFG Union Bank N.A | Senior debt obligations | Floating Rate Debt Due December 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	299			300
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	8,508			8,909
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate 2.10 Percent Notes due December 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 698			$ 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 [Member]
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	1.60%			2.49%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 [Member] | 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 [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
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 [Member]
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.92%			1.90%
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate 3.15 Percent Notes Due April 2022 [Member]
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 July 2019 and December 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 5,100			$ 5,500
Debt instrument, fixed interest rate (as a percent)	2.83%			2.95%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate Debt Due December 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.72%			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	555			598
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Floating Rate Borrowings Due Between December 2020 and May 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 250			$ 250
Long-term debt, weighted average interest rate (as a percent)	1.45%			1.94%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between September 2019 and May 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 109			$ 137
Long-term debt, weighted average interest rate (as a percent)	2.29%			2.22%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between September 2019 and May 2024 [Member] | Minimum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	1.71%			1.68%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between September 2019 and May 2024 [Member] | 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 3.05% at March 31, 2020 and 3.93% at December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 17			$ 13
Long-term debt, weighted average interest rate (as a percent)	3.05%			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 March 31, 2020 and 5.34% December 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 29			$ 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 [Member]
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	2.17%			2.17%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between December 2019 and July 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 147			$ 166
Long-term debt, weighted average interest rate (as a percent)	3.17%			3.07%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between December 2019 and July 2023 [Member] | 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 [Member] | 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 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 3			$ 3