Long-Term Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 17,918	$ 17,918	$ 12,162
Repayments of Long-term Debt		9,655	4,801	$ 3,082
Federal Home Loan advances, collateral pledged	44,000	44,000	43,000
Senior Debt Obligations [Member] | Fixed rate 2.25% notes due February 2020
Debt Instrument [Line Items]
Repayments of Long-term Debt	1,000
Total Loss Absorbing Capacity Loan Agreement [Member]
Debt Instrument [Line Items]
Long-term debt	6,500	6,500
MUFG Americas Holdings Corporation
Debt Instrument [Line Items]
Long-term debt	7,355	$ 7,355	6,997
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member]
Debt Instrument [Line Items]
Redeemable debt, percentage of par value		100.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate senior notes due February 2018. These notes, which bore interest at 0.57% above 3-month LIBOR, had a rate of 1.97% at December 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 250
Debt instrument, effective interest rate	0.00%	0.00%	197.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate senior notes due February 2018. These notes, which bore interest at 0.57% above 3-month LIBOR, had a rate of 1.97% at December 31, 2017 | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		0.00%	57.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Fixed rate 1.625% notes due February 2018
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 450
Debt instrument, stated rate	0.00%	0.00%	163.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Fixed rate 2.25% notes due February 2020
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 998
Debt instrument, stated rate	0.00%	0.00%	225.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Fixed rate 3.50% notes due June 2022
Debt Instrument [Line Items]
Long-term debt	$ 398	$ 398	$ 398
Debt instrument, stated rate	350.00%	350.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Fixed rate 3.00% notes due February 2025
Debt Instrument [Line Items]
Long-term debt	$ 397	$ 397	496
Debt instrument, stated rate	300.00%	300.00%
Repayments of Long-term Debt	$ 100
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate debt due March 2020. This note, which bore interest at 0.86% above 3-month LIBOR, had a rate of 2.45% at December 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 545		$ 3,500
Debt instrument, effective interest rate			245.00%
Debt instrument, stated rate	0.00%	0.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate debt due March 2020. This note, which bore interest at 0.86% above 3-month LIBOR, had a rate of 2.45% at December 31, 2017 | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		86.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due September 2020 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 3,500
Debt instrument, effective interest rate			254.00%
Interest rate above variable interest rate (as a percent)		0.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due September 2020 [Member] | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		85.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due December 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	1,625	$ 1,625	$ 0
Debt instrument, effective interest rate			0.00%
Interest rate above variable interest rate (as a percent)		358.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due December 2021 [Member] | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		81.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due December 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	3,250	$ 3,250	$ 0
Debt instrument, effective interest rate			0.00%
Interest rate above variable interest rate (as a percent)		367.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due December 2022 [Member] | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		90.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due December 2023 LIBOR [Member]
Debt Instrument [Line Items]
Long-term debt	1,625	$ 1,625	$ 0
Debt instrument, effective interest rate			0.00%
Interest rate above variable interest rate (as a percent)		376.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating Rate Debt Due December 2023 LIBOR [Member] | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		99.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate subordinated debt due December 2023. This note, which bore interest at 1.38% above 3-month LIBOR, had a rate of 3.07% at December 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 24	$ 24	$ 24
Debt instrument, effective interest rate	76.00%	76.00%	76.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate subordinated debt due December 2023. This note, which bore interest at 1.38% above 3-month LIBOR, had a rate of 3.07% at December 31, 2017 | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		138.00%
MUFG Americas Holdings Corporation | Senior Debt Obligations [Member] | Floating rate subordinated debt due December 2023. This note, which bore interest at 1.38% above 3-month LIBOR, had a rate of 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)		76.00%
MUFG Americas Holdings Corporation | Subordinated Debt | Floating Rate Surbordinated Debt Due December 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 300
MUFG Americas Holdings Corporation | Subordinated Debt | Floating rate subordinated debt due December 2023. This note, which bore interest at 1.38% above 3-month LIBOR, had a rate of 3.07% at December 31, 2017
Debt Instrument [Line Items]
Debt instrument, effective interest rate	0.00%	0.00%	307.00%
MUFG Americas Holdings Corporation | Junior subordinated debt payable to trusts | Floating rate note due September 2036 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 36	$ 36	$ 36
Debt instrument, effective interest rate	449.00%	449.00%	329.00%
MUB
Debt Instrument [Line Items]
Long-term debt	$ 9,632	$ 9,632	$ 3,810
Amount available for issuance under the bank note program	5,900	5,900
MUB | Maximum
Debt Instrument [Line Items]
Amount available for issuance under the bank note program	12,000	$ 12,000
MUB | Senior Debt Obligations [Member]
Debt Instrument [Line Items]
Amount available for issuance under the Bank note program		1 year
MUB | Senior Debt Obligations [Member] | Fixed Rate Advances Due Between January 2019 and December 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 9,100	$ 9,100	$ 1,500
Debt instrument, stated rate	266.00%	266.00%	151.00%
MUB | Senior Debt Obligations [Member] | Fixed rate 2.625% notes due September 2018
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 1,000
Debt instrument, stated rate	263.00%	263.00%
MUB | Senior Debt Obligations [Member] | Fixed rate 2.25% notes due May 2019
Debt Instrument [Line Items]
Long-term debt	$ 498	$ 498	$ 497
Debt instrument, stated rate	225.00%	225.00%
MUB | Senior Debt Obligations [Member] | Floating rate subordinated debt due June 2023. This note, which bore interest at 1.20% above 3-month LIBOR, had a rate of 2.89% at December 31, 2017
Debt Instrument [Line Items]
Debt instrument, effective interest rate	0.00%	0.00%	289.00%
MUB | Senior Debt Obligations [Member] | Floating rate subordinated debt due June 2023. This note, which bore interest at 1.20% above 3-month LIBOR, had a rate of 2.89% at December 31, 2017 | LIBOR
Debt Instrument [Line Items]
Interest rate above variable interest rate (as a percent)		120.00%
MUB | Subordinated Debt
Debt Instrument [Line Items]
Amount available for issuance under the Bank note program		5 years
MUB | Subordinated Debt | Floating rate subordinated debt due June 2023. This note, which bore interest at 1.20% above 3-month LIBOR, had a rate of 2.89% at December 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 0	$ 0	$ 750
MUB | Other
Debt Instrument [Line Items]
Long-term debt	34	34	63
Other Subsidiaries, Excluding MUFG Bank, NA [Member]
Debt Instrument [Line Items]
Long-term debt	931	931	1,355
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Floating Rate Borrowings Due Between December 2020 and May 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 250	$ 250	$ 291
Debt instrument, effective interest rate	280.00%	280.00%	178.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Fixed Rate Borrowings Due Between February 2019 and May 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 244	$ 244	$ 339
Debt instrument, effective interest rate	182.00%	182.00%	212.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Fixed Rate Borrowings Due Between February 2019 and May 2024 [Member] | Minimum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	14.00%	14.00%	137.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Fixed Rate Borrowings Due Between February 2019 and May 2024 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	244.00%	244.00%	265.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Floating Rate Debt Due March 2019 [Member]
Debt Instrument [Line Items]
Debt instrument, effective interest rate			295.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Floating Rate Debt Due March 2019 [Member] | Minimum
Debt Instrument [Line Items]
Debt instrument, effective interest rate			288.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Senior Debt Obligations [Member] | Floating Rate Debt Due March 2019 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument, effective interest rate			304.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Subordinated Debt | Floating Rate Debt Due March 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 75	$ 75	$ 185
Debt instrument, effective interest rate	413.00%	413.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Subordinated Debt | Floating Rate Debt Due March 2019 [Member] | Minimum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	0.00%	0.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Subordinated Debt | Floating Rate Debt Due March 2019 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	0.00%	0.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Floating Rate Nonrecourse Debt Due Between February 2019 and December 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 53	$ 53	$ 79
Debt instrument, effective interest rate	409.00%	409.00%	307.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Floating Rate Nonrecourse Debt Due Between February 2019 and December 2021 [Member] | Minimum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	275.00%	275.00%	149.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Floating Rate Nonrecourse Debt Due Between February 2019 and December 2021 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	417.00%	417.00%	558.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Fixed Rate Nonrecourse Debt Due Between January 2019 and July 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 187	$ 187	$ 240
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Floating Rate Nonrecourse Borrowings Due Between February 2019 and May 2019 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 89	$ 89	$ 185
Debt instrument, effective interest rate	421.00%	421.00%	288.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Floating Rate Nonrecourse Borrowings Due Between February 2019 and May 2019 [Member] | Minimum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	417.00%	417.00%	250.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Floating Rate Nonrecourse Borrowings Due Between February 2019 and May 2019 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument, effective interest rate	448.00%	448.00%	354.00%
Other Subsidiaries, Excluding MUFG Bank, NA [Member] | Secured Debt | Fixed rate non-recourse borrowings due December 2026 which had an interest rate of 5.34% at December 31, 2018 and December 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 33	$ 33	$ 36
Debt instrument, effective interest rate	534.00%	534.00%