Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Mar. 31, 2019
Debt Instrument [Line Items]
Long-term debt	$ 16,068		$ 17,918
Proceeds from issuance of long-term debt	1,703	$ 4,050
Parent Company [Member]
Debt Instrument [Line Items]
Long-term debt	7,356		7,355
Parent Company [Member] | Senior debt obligations | Fixed rate 3.50% notes due June 2022
Debt Instrument [Line Items]
Long-term debt	$ 398		$ 398
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	$ 397		$ 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% at June 30, 2019 and 0.76% at December 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 24		$ 24
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,625		$ 1,625
Long-term debt, weighted average interest rate (as a percent)	3.24%		3.58%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2021 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)	0.81%		0.81%
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)	3.33%		3.67%
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 2023 LIBOR [Member]
Debt Instrument [Line Items]
Long-term debt	$ 1,625		$ 1,625
Long-term debt, weighted average interest rate (as a percent)	3.42%		3.76%
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] | 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 0% at June 30, 2019 and 4.49% at December 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 37		$ 36
Long-term debt, weighted average interest rate (as a percent)	4.11%		4.49%
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	$ 8,033
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	299		$ 0	$ 300
MUFG Union Bank N.A
Debt Instrument [Line Items]
Long-term debt	$ 8,033		9,632
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)	3.07%
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%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate 2.25% notes due May 2019
Debt Instrument [Line Items]
Long-term debt	$ 0		$ 498
Debt instrument, fixed interest rate (as a percent)	0.00%		2.25%
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	$ 997		$ 0	$ 1,000
Debt instrument, fixed interest rate (as a percent)	3.15%		0.00%	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	$ 6,700		$ 9,100
Debt instrument, fixed interest rate (as a percent)	2.84%		2.66%
MUFG Union Bank N.A | Other
Debt Instrument [Line Items]
Long-term debt	$ 37		$ 34
Other Subsidiaries, Excluding MUFG Bank, NA
Debt Instrument [Line Items]
Long-term debt	$ 679		$ 931
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between June 2019 and May 2024 with a weighted average interest rate of 0% (between 0.14% and 2.44%) at June 30, 2019 and 1.82% (between 0.14% and 2.44%) at December 31, 2018 | Minimum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	0.14%		0.14%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between June 2019 and May 2024 with a weighted average interest rate of 0% (between 0.14% and 2.44%) at June 30, 2019 and 1.82% (between 0.14% and 2.44%) at December 31, 2018 | 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 | 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)	2.33%		2.80%
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	$ 205		$ 244
Long-term debt, weighted average interest rate (as a percent)	1.79%		1.82%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Floating Rate Debt Due March 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)			4.13%
Other Subsidiaries, Excluding MUFG Bank, NA | Subordinated Debt | Floating Rate Debt Due March 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0		$ 75
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between May 2019 and February 2021. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 4.64% (between 4.59% and 4.66%) at June 30, 2019 and 4.21% (between 4.17% and 4.48%) at December 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 16		$ 89
Long-term debt, weighted average interest rate (as a percent)	4.60%		4.21%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between May 2019 and February 2021. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 4.64% (between 4.59% and 4.66%) at June 30, 2019 and 4.21% (between 4.17% and 4.48%) at December 31, 2018 | Minimum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	4.51%		4.20%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between May 2019 and February 2021. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 4.64% (between 4.59% and 4.66%) at June 30, 2019 and 4.21% (between 4.17% and 4.48%) at December 31, 2018 | Maximum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)	4.70%		4.48%
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 June 30, 2019 and 5.34% at December 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 31		$ 33
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)	2.65%		4.09%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt December 2021 [Member] | Minimum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)			2.75%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt December 2021 [Member] | Maximum
Debt Instrument [Line Items]
Long-term debt, weighted average interest rate (as a percent)			4.17%
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	$ 174		$ 187
Long-term debt, weighted average interest rate (as a percent)	3.05%		3.05%
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%
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%
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		$ 53