Long-Term Debt (Details)		3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 loan	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument
Long-term debt				$ 11,419,000,000	$ 11,410,000,000
Parent Company
Debt Instrument
Long-term debt				6,995,000,000	3,470,000,000
Parent Company | Senior debt obligations | Floating rate senior notes due February 2018. These notes, which bear interest at 0.57% above 3-month LIBOR, had a rate of 1.88% at September 30, 2017 and 1.46% at December 31, 2016
Debt Instrument
Long-term debt				$ 250,000,000	$ 250,000,000
Long-term debt, weighted average interest rate (as a percent)				1.88%	1.46%
Parent Company | Senior debt obligations | Floating rate senior notes due February 2018. These notes, which bear interest at 0.57% above 3-month LIBOR, had a rate of 1.88% at September 30, 2017 and 1.46% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				0.57%	0.57%
Parent Company | Senior debt obligations | Fixed rate 1.625% notes due February 2018
Debt Instrument
Long-term debt				$ 450,000,000	$ 449,000,000
Debt instrument, fixed interest rate (as a percent)				1.625%	1.625%
Parent Company | Senior debt obligations | Fixed rate 2.25% notes due February 2020
Debt Instrument
Long-term debt				$ 997,000,000	$ 997,000,000
Debt instrument, fixed interest rate (as a percent)				2.25%	2.25%
Parent Company | Senior debt obligations | Fixed rate 3.50% notes due June 2022
Debt Instrument
Long-term debt				$ 397,000,000	$ 397,000,000
Debt instrument, fixed interest rate (as a percent)				3.50%	3.50%
Parent Company | Senior debt obligations | Fixed rate 3.00% notes due February 2025
Debt Instrument
Long-term debt				$ 496,000,000	$ 496,000,000
Debt instrument, fixed interest rate (as a percent)				3.00%	3.00%
Parent Company | Senior debt obligations | Floating rate debt due September 2020. This note, which bears interest at 0.85% above 3-month LIBOR, had a rate of 2.18% at September 30, 2017
Debt Instrument
Long-term debt				$ 3,500,000,000	$ 0
Long-term debt, weighted average interest rate (as a percent)				2.18%
Amount of debt borrowed		$ 3,500,000,000
Prepayment of long term debt				$ 500,000
Parent Company | Senior debt obligations | Floating rate debt due September 2020. This note, which bears interest at 0.85% above 3-month LIBOR, had a rate of 2.18% at September 30, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				0.85%
Parent Company | 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 September 30, 2017
Debt Instrument
Long-term debt				$ 24,000,000	0
Long-term debt, weighted average interest rate (as a percent)				0.76%
Parent Company | 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 September 30, 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				0.76%
Parent Company | Senior debt obligations | Floating rate debt due March 2020. This note, which bears interest at 0.86% above 3-month LIBOR, had a rate of 2.18% at September 30, 2017 and 1.82% at December 31, 2016
Debt Instrument
Long-term debt				$ 545,000,000	$ 545,000,000
Long-term debt, weighted average interest rate (as a percent)				2.18%	1.82%
Parent Company | Senior debt obligations | Floating rate debt due March 2020. This note, which bears interest at 0.86% above 3-month LIBOR, had a rate of 2.18% at September 30, 2017 and 1.82% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				0.86%
Parent Company | 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 2.71% at September 30, 2017 and 2.38% at December 31, 2016
Debt Instrument
Long-term debt				$ 300,000,000	$ 300,000,000
Long-term debt, weighted average interest rate (as a percent)				2.71%	2.38%
Parent Company | 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 2.71% at September 30, 2017 and 2.38% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				1.38%
Parent Company | Junior subordinated debt payable to trusts | Floating rate note due September 2036. This note had an interest rate of 3.02% at September 30, 2017 and 2.66% at December 31, 2016
Debt Instrument
Long-term debt				$ 36,000,000	$ 36,000,000
Long-term debt, weighted average interest rate (as a percent)				3.02%	2.66%
MUFG Union Bank N.A
Debt Instrument
Long-term debt				$ 2,906,000,000	$ 6,557,000,000
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due September 2020. This note, which bears interest at 0.85% above 3-month LIBOR, had a rate of 2.18% at September 30, 2017 | MUFG Americas Holding Corporation
Debt Instrument
Number of loans from BTMU prepaid | loan			3
Repayments of debt	$ 3,500,000,000
MUFG Union Bank N.A | Senior debt obligations | Floating rate notes due May 2017. These notes, which bear interest at 0.40% above 3-month LIBOR, had a rate of 1.28% at December 31, 2016
Debt Instrument
Long-term debt				$ 0	$ 250,000,000
Long-term debt, weighted average interest rate (as a percent)					1.28%
MUFG Union Bank N.A | Senior debt obligations | Floating rate notes due May 2017. These notes, which bear interest at 0.40% above 3-month LIBOR, had a rate of 1.28% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				0.40%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate 2.125% notes due June 2017
Debt Instrument
Long-term debt				$ 0	$ 500,000,000
Debt instrument, fixed interest rate (as a percent)					2.125%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate 2.625% notes due September 2018
Debt Instrument
Long-term debt				$ 999,000,000	$ 999,000,000
Debt instrument, fixed interest rate (as a percent)				2.625%	2.625%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate FHLB of San Francisco advances due between May 2018 and August 2018. These notes bear a combined weighted-average rate of 1.33% at September 30, 2017
Debt Instrument
Long-term debt				$ 600,000,000	$ 0
Debt instrument, fixed interest rate (as a percent)				1.33%
MUFG Union Bank N.A | Senior debt obligations | Fixed rate 2.250% notes due May 2019
Debt Instrument
Long-term debt				$ 499,000,000	$ 500,000,000
Debt instrument, fixed interest rate (as a percent)				2.25%	2.25%
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due January 2018. This note, which bears interest at 0.85% above 1-month LIBOR, had a rate of 1.47% at December 31, 2016
Debt Instrument
Long-term debt				$ 0	$ 1,000,000,000
Long-term debt, weighted average interest rate (as a percent)					1.47%
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due January 2018. This note, which bears interest at 0.85% above 1-month LIBOR, had a rate of 1.47% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				0.85%
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due January 2018. This note, which bears interest at 0.87% above 1-month LIBOR, had a rate of 1.49% at December 31, 2016
Debt Instrument
Long-term debt				$ 0	$ 1,500,000,000
Long-term debt, weighted average interest rate (as a percent)					1.49%
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due January 2018. This note, which bears interest at 0.87% above 1-month LIBOR, had a rate of 1.49% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)					0.87%
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due January 2018. This note, which bears interest at 1.03% above 1-month LIBOR, had a rate of 1.65% at December 31, 2016
Debt Instrument
Long-term debt				0	$ 1,000,000,000
Long-term debt, weighted average interest rate (as a percent)					1.65%
MUFG Union Bank N.A | Senior debt obligations | Floating rate debt due January 2018. This note, which bears interest at 1.03% above 1-month LIBOR, had a rate of 1.65% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)					1.03%
MUFG Union Bank N.A | Subordinated Debt | Floating rate subordinated debt due June 2023. This note, which bears interest at 1.20% above 3-month LIBOR, had a rate of 2.53% at September 30, 2017 and 2.20% at December 31, 2016
Debt Instrument
Long-term debt				$ 750,000,000	$ 750,000,000
Long-term debt, weighted average interest rate (as a percent)				2.53%	2.20%
MUFG Union Bank N.A | Subordinated Debt | Floating rate subordinated debt due June 2023. This note, which bears interest at 1.20% above 3-month LIBOR, had a rate of 2.53% at September 30, 2017 and 2.20% at December 31, 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)				1.20%
MUFG Union Bank N.A | Other
Debt Instrument
Long-term debt				$ 58,000,000	$ 58,000,000
Other Subsidiaries, Excluding MUFG Bank, NA
Debt Instrument
Long-term debt				1,518,000,000	1,383,000,000
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between March 2019 and May 2021. These notes, which bear interest above 3-month LIBOR had a weighted-average interest rate of 1.41% at September 30, 2017 and 0.99% at December 31, 2016
Debt Instrument
Long-term debt				$ 270,000,000	$ 250,000,000
Long-term debt, weighted average interest rate (as a percent)				1.41%	0.99%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and June 2023 with a weighted-average interest rate of 2.10% (between 1.37% and 2.65%) at September 30, 2017 and 2.15% (between 1.71% and 2.44%) at December 31, 2016
Debt Instrument
Long-term debt				$ 340,000,000	$ 384,000,000
Long-term debt, weighted average interest rate (as a percent)				2.10%	2.15%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and June 2023 with a weighted-average interest rate of 2.10% (between 1.37% and 2.65%) at September 30, 2017 and 2.15% (between 1.71% and 2.44%) at December 31, 2016 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				1.37%	1.71%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various fixed rate borrowings due between February 2019 and June 2023 with a weighted-average interest rate of 2.10% (between 1.37% and 2.65%) at September 30, 2017 and 2.15% (between 1.71% and 2.44%) at December 31, 2016 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.65%	2.44%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between March 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 2.95% (between 2.88% and 3.04%) at September 30, 2017 and 2.68% (between 2.61% and 2.77%) at December 31, 2016
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.95%	2.68%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between March 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 2.95% (between 2.88% and 3.04%) at September 30, 2017 and 2.68% (between 2.61% and 2.77%) at December 31, 2016 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.88%	2.61%
Other Subsidiaries, Excluding MUFG Bank, NA | Senior debt obligations | Various floating rate borrowings due between March 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 2.95% (between 2.88% and 3.04%) at September 30, 2017 and 2.68% (between 2.61% and 2.77%) at December 31, 2016 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				3.04%	2.77%
Other Subsidiaries, Excluding MUFG Bank, NA | Subordinated Debt | Various floating rate borrowings due between March 2018 and March 2019. These notes, which bear interest above 6-month LIBOR had a weighted-average interest rate of 2.95% (between 2.88% and 3.04%) at September 30, 2017 and 2.68% (between 2.61% and 2.77%) at December 31, 2016
Debt Instrument
Long-term debt				$ 185,000,000	$ 185,000,000
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate nonrecourse borrowings due to BTMU between March 2018 and July 2023. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.32% (between 1.49% and 5.58%) at September 30, 2017 and 1.67% (between 0.25% and 2.41%) at December 31, 2016
Debt Instrument
Long-term debt				$ 247,000,000	$ 127,000,000
Long-term debt, weighted average interest rate (as a percent)				3.32%	1.67%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate nonrecourse borrowings due to BTMU between March 2018 and July 2023. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.32% (between 1.49% and 5.58%) at September 30, 2017 and 1.67% (between 0.25% and 2.41%) at December 31, 2016 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				1.49%	0.25%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate nonrecourse borrowings due to BTMU between March 2018 and July 2023. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 3.32% (between 1.49% and 5.58%) at September 30, 2017 and 1.67% (between 0.25% and 2.41%) at December 31, 2016 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				5.58%	2.41%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed rate nonrecourse borrowings due to BTMU between January 2019 and March 2023 which had an interest rate of 2.70% at September 30, 2017
Debt Instrument
Long-term debt				$ 82,000,000	$ 0
Long-term debt, weighted average interest rate (as a percent)				2.70%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Fixed rate nonrecourse borrowings due December 2026 which had an interest rate of 5.34% at September 30, 2017 and December 31, 2016
Debt Instrument
Long-term debt				$ 37,000,000	$ 39,000,000
Debt instrument, fixed interest rate (as a percent)				5.34%	5.34%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate nonrecourse borrowings due between October 2017 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 2.54% (between 2.08% and 3.35%) at September 30, 2017 and 2.04% (between 0.85% and 2.73%) at December 31, 2016
Debt Instrument
Long-term debt				$ 357,000,000	$ 398,000,000
Long-term debt, weighted average interest rate (as a percent)				2.54%	2.04%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate nonrecourse borrowings due between October 2017 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 2.54% (between 2.08% and 3.35%) at September 30, 2017 and 2.04% (between 0.85% and 2.73%) at December 31, 2016 | Minimum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				2.08%	0.85%
Other Subsidiaries, Excluding MUFG Bank, NA | Secured Debt | Various floating rate nonrecourse borrowings due between October 2017 and May 2019. These notes, which bear interest above 1- or 3-month LIBOR had a weighted-average interest rate of 2.54% (between 2.08% and 3.35%) at September 30, 2017 and 2.04% (between 0.85% and 2.73%) at December 31, 2016 | Maximum
Debt Instrument
Long-term debt, weighted average interest rate (as a percent)				3.35%	2.73%