Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term debt	$ 14,145			$ 14,631
Proceeds from issuance of long-term debt	2,140	$ 613
Parent Company [Member]
Debt Instrument [Line Items]
Long-term debt	7,329			7,276
Parent Company [Member] | Fixed rate 3.50% notes due June 2022 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 400			$ 399
Debt instrument, fixed interest rate (as a percent)	3.50%			3.50%
Parent Company [Member] | Fixed rate 3.00% notes due February 2025 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 398			$ 398
Debt instrument, fixed interest rate (as a percent)	3.00%			3.00%
Parent Company [Member] | 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 June 30, 2021 and 0.76% at December 31, 2020 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 26			$ 27
Parent Company [Member] | 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 | Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	0.76%			0.76%
Parent Company [Member] | 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) | Subordinated Debt
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.76%		0.76%
Parent Company [Member] | Floating rate note due September 2036. This note had an interest rate of 1.92% at December 31, 2020 | Junior subordinated debt payable to trusts
Debt Instrument [Line Items]
Long-term debt	$ 0			$ 37
Long-term debt, weighted average interest rate (as a percent)	0.00%			1.92%
Parent Company [Member] | Floating Rate Debt Due December 2023 LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 1,625			$ 1,625
Long-term debt, weighted average interest rate (as a percent)	1.10%			1.23%
Parent Company [Member] | Floating Rate Debt Due December 2023 LIBOR | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.99%		0.99%
Parent Company [Member] | Floating Rate Debt Due December 2023 LIBOR Issued Q4 2019 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 1,765			$ 1,765
Long-term debt, weighted average interest rate (as a percent)	1.05%			1.15%
Parent Company [Member] | Floating Rate Debt Due December 2023 LIBOR Issued Q4 2019 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.94%		0.94%
Parent Company [Member] | Floating Rate Debt Due March 2024 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 775			$ 775
Long-term debt, weighted average interest rate (as a percent)	0.88%			0.98%
Parent Company [Member] | Floating Rate Debt Due March 2024 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.76%		0.76%
Parent Company [Member] | Floating Rate Debt Due June 2024 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 750			$ 750
Long-term debt, weighted average interest rate (as a percent)	0.91%			1.00%
Parent Company [Member] | Floating Rate Debt Due June 2024 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.79%		0.79%
Parent Company [Member] | Floating Rate Debt Due September 2024 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 750			$ 750
Long-term debt, weighted average interest rate (as a percent)	0.94%			1.03%
Parent Company [Member] | Floating Rate Debt Due September 2024 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.82%		0.82%
Parent Company [Member] | Floating Rate Debt Due December 2024 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 750			$ 750
Long-term debt, weighted average interest rate (as a percent)	0.96%			1.06%
Parent Company [Member] | Floating Rate Debt Due December 2024 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.84%		0.84%
Parent Company [Member] | Floating Rate Debt Due March 2024 Issued Q1 2021 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 90			$ 0
Long-term debt, weighted average interest rate (as a percent)	0.78%			0.00%
Parent Company [Member] | Floating Rate Debt Due March 2024 Issued Q1 2021 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.66%		0.66%
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	$ 6,384
MUFG Union Bank N.A | Floating Rate debt due March 2022 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	300			$ 300
MUFG Union Bank N.A | Floating Rate Debt Due December 2022 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	300			300
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	6,384			6,884
MUFG Union Bank N.A | Other
Debt Instrument [Line Items]
Long-term debt	13			12
MUFG Union Bank N.A | Fixed Rate 2.10 Percent Notes due December 2022 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 699			$ 699
Debt instrument, fixed interest rate (as a percent)	2.10%			2.10%
MUFG Union Bank N.A | Floating Rate debt due March 2022 | Senior debt obligations
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.73%			0.83%
MUFG Union Bank N.A | Floating Rate debt due March 2022 | London Interbank Offered Rate LIBOR | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.60%		0.60%
MUFG Union Bank N.A | Fixed Rate 3.15 Percent Notes Due April 2022 | Senior debt obligations
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 | Floating Rate Debt Due December 2022 | Senior debt obligations
Debt Instrument [Line Items]
Debt instrument, fixed interest rate (as a percent)	0.76%			0.78%
MUFG Union Bank N.A | Floating Rate Debt Due December 2022 | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Senior debt obligations
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.71%		0.71%
MUFG Union Bank N.A | Fixed Rate Advances Due Between November 2021 and September 2026 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 4,074			$ 4,575
Debt instrument, fixed interest rate (as a percent)	2.37%			2.76%
Other Subsidiaries, Excluding MUFG Bank, NA
Debt Instrument [Line Items]
Long-term debt	$ 432			$ 471
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed rate 5.34% non-recourse borrowings due December 2026 | Secured Debt
Debt Instrument [Line Items]
Long-term debt	$ 24			$ 26
Debt instrument, fixed interest rate (as a percent)	5.34%			5.34%
Other Subsidiaries, Excluding MUFG Bank, NA | Floating Rate Nonrecourse Debt December 2021 | Secured Debt
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	1.01%			0.51%
Other Subsidiaries, Excluding MUFG Bank, NA | Floating Rate Nonrecourse Debt Due Between February 2019 and December 2021 | Secured Debt
Debt Instrument [Line Items]
Long-term debt	$ 3			$ 3
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed Rate Borrowings Due Between August 2021 and September 2027 | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt	$ 312			$ 323
Long-term debt, weighted average interest rate (as a percent)	1.45%			1.48%
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed Rate Borrowings Due Between August 2021 and September 2027 | Minimum | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	1.23%			1.23%
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed Rate Borrowings Due Between August 2021 and September 2027 | Maximum | Senior debt obligations
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	2.44%			2.44%
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed Rate Nonrecourse Debt Due Between July 2022 and July 2023 | Secured Debt
Debt Instrument [Line Items]
Long-term debt	$ 93			$ 119
Long-term debt, weighted average interest rate (as a percent)	3.20%			3.15%
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed Rate Nonrecourse Debt Due Between July 2022 and July 2023 | Minimum | Secured Debt
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	2.18%			1.96%
Other Subsidiaries, Excluding MUFG Bank, NA | Fixed Rate Nonrecourse Debt Due Between July 2022 and July 2023 | Maximum | Secured Debt
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	3.72%			3.72%