Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt Instrument
Long-term debt	$ 17,335		$ 17,918
Proceeds from issuance of long-term debt	1,300	$ 2,755
Parent Company [Member]
Debt Instrument
Long-term debt	7,356		7,355
Parent Company [Member] | Senior debt obligations | Fixed rate 3.50% notes due June 2022
Debt Instrument
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
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 .76% at September 30, 2018 and 0.76% at December 31, 2017
Debt Instrument
Long-term debt	$ 24		$ 24
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2021 [Member]
Debt Instrument
Long-term debt	$ 1,625		$ 1,625
Long-term debt, weighted average interest rate (as a percent)	3.40%		3.58%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2021 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
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
Long-term debt	$ 3,250		$ 3,250
Long-term debt, weighted average interest rate (as a percent)	3.49%		3.67%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2022 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
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
Long-term debt	$ 1,625		$ 1,625
Long-term debt, weighted average interest rate (as a percent)	3.58%		3.76%
Parent Company [Member] | Senior debt obligations | Floating Rate Debt Due December 2023 LIBOR [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)	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
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
Interest rate above variable interest rate (as a percent)	0.76%
Parent Company [Member] | Junior subordinated debt payable to trusts | Floating rate note due September 2036. This note had an interest rate of 4.03% at September 30, 2018 and 3.29% at December 31, 2017
Debt Instrument
Long-term debt	$ 37		$ 36
Long-term debt, weighted average interest rate (as a percent)	4.31%		4.49%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 [Member]
Debt Instrument
Long-term debt	$ 299		$ 0
MUFG Union Bank N.A
Debt Instrument
Long-term debt	$ 9,280		9,632
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 [Member]
Debt Instrument
Debt instrument, fixed interest rate (as a percent)	3.21%
MUFG Union Bank N.A | Senior debt obligations | Floating Rate debt due March 2022 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
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
Long-term debt	$ 499		$ 498
Debt instrument, fixed interest rate (as a percent)	2.25%		2.25%
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate 3.15 Percent Notes Due April 2022 [Member]
Debt Instrument
Long-term debt	$ 996		$ 0
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 April 2019 and December 2023 [Member]
Debt Instrument
Debt instrument, fixed interest rate (as a percent)	2.79%		2.66%
MUFG Union Bank N.A | Senior debt obligations | Fixed Rate Advances Due Between January 2019 and December 2023 [Member]
Debt Instrument
Long-term debt	$ 7,450		$ 9,100
MUFG Union Bank N.A | Other
Debt Instrument
Long-term debt	36		34
Other Subsidiaries, Excluding MUFG Bank, NA
Debt Instrument
Long-term debt	$ 699		$ 931
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and May 2024 with a weighted average interest rate of 1.82% (between 0.14% and 2.44%) at September 30, 2018 and 2.12% (between 1.37% and 2.65%) at December 31, 2017
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)	1.80%		1.82%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and May 2024 with a weighted average interest rate of 1.82% (between 0.14% and 2.44%) at September 30, 2018 and 2.12% (between 1.37% and 2.65%) at December 31, 2017 | Minimum
Debt Instrument
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 February 2019 and May 2024 with a weighted average interest rate of 1.82% (between 0.14% and 2.44%) at September 30, 2018 and 2.12% (between 1.37% and 2.65%) at December 31, 2017 | Maximum
Debt Instrument
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
Long-term debt	$ 250		$ 250
Long-term debt, weighted average interest rate (as a percent)	2.60%		2.80%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Fixed Rate Borrowings Due Between February 2019 and May 2024 [Member]
Debt Instrument
Long-term debt	$ 223		$ 244
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Floating Rate Debt Due March 2019 [Member]
Debt Instrument
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
Long-term debt	0		$ 75
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between February 2019 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 3.64% (between 3.34% and 4.23%) at September 30, 2018 and 2.88% (between 2.50% and 3.54%) at December 31, 2017
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)		4.64%	4.21%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between February 2019 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 3.64% (between 3.34% and 4.23%) at September 30, 2018 and 2.88% (between 2.50% and 3.54%) at December 31, 2017 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)		4.59%	4.20%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate non-recourse borrowings due between February 2019 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted average interest rate of 3.64% (between 3.34% and 4.23%) at September 30, 2018 and 2.88% (between 2.50% and 3.54%) at December 31, 2017 | Maximum
Debt Instrument
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 September 30, 2018 and December 31, 2017
Debt Instrument
Long-term debt	$ 32		$ 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
Long-term debt, weighted average interest rate (as a percent)	2.74%		4.09%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt December 2021 [Member] | Minimum
Debt Instrument
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
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
Long-term debt, weighted average interest rate (as a percent)		3.04%	3.05%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Debt Due Between February 2019 and December 2021 [Member]
Debt Instrument
Long-term debt	$ 3		$ 53
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed Rate Nonrecourse Debt Due Between January 2019 and July 2023 [Member]
Debt Instrument
Long-term debt	178		187
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Floating Rate Nonrecourse Borrowings Due Between February 2019 and May 2019 [Member]
Debt Instrument
Long-term debt	$ 13		$ 89