Long-Term Debt (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Debt Instrument
Long-term debt		$ 11,843	$ 12,349
MUFG Americas Holdings Corporation
Debt Instrument
Long-term debt		2,922	2,932
MUFG Americas Holdings Corporation | Senior debt | Floating Rate Senior Notes due February 2018
Debt Instrument
Long-term Debt	[1]	249	250
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Senior Note 1.625 Percent due February 2018
Debt Instrument
Long-term Debt	[1]	449	450
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Senior Note 3.50 Percent due June 2022
Debt Instrument
Long-term Debt		397	398
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Senior Note 2.25 Percent due February 2022
Debt Instrument
Long-term Debt	[1]	996	1,000
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Senior Note 3.00 Percent due February 2025
Debt Instrument
Long-term Debt	[1]	$ 495	$ 498
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate One Point Six Two Five Percent Notes Due February Two Thousand Eighteen [Member] [Domain] [Domain]
Debt Instrument
Debt instrument, Fixed interest rate (as a percent)		1.625%	1.625%
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Two Point Two Five Percent Notes Due February Two Thousand Twenty [Member] [Domain] [Domain]
Debt Instrument
Debt instrument, Fixed interest rate (as a percent)		2.25%	2.25%
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Three Point Five Zero Percent Notes Due June Two Thousand Twenty Two [Member]
Debt Instrument
Debt instrument, Fixed interest rate (as a percent)		3.50%	3.50%
MUFG Americas Holdings Corporation | Senior debt | Fixed Rate Three Point Zero Percent Notes Due February Two Thousand Twenty Five [Member] [Domain]
Debt Instrument
Debt instrument, Fixed interest rate (as a percent)		3.00%	3.00%
MUFG Americas Holdings Corporation | Subordinated Debt | Floating rate subordinated debt due December 2023
Debt Instrument
Long-term Debt		$ 300	$ 300
Long-term Debt, Weighted Average Interest Rate		2.01%	1.98%
MUFG Americas Holdings Corporation | Subordinated Debt | Floating rate subordinated debt due December 2023 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		1.38%	1.38%
MUFG Americas Holdings Corporation | Junior subordinated debt payable to trusts | Floating rate
Debt Instrument
Long-term Debt	[2]	$ 36	$ 36
Long-term Debt, Weighted Average Interest Rate		2.33%	2.21%
Union Bank, N.A. and other subsidiaries
Debt Instrument
Long-term debt		$ 8,921	$ 9,417
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Senior Notes due February 2018
Debt Instrument
Long-term Debt, Weighted Average Interest Rate		1.19%	0.91%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Senior Notes due February 2018 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.57%	0.57%
Union Bank, N.A. and other subsidiaries | Senior debt | Federal Home Loan Bank Advances
Debt Instrument
Long-term Debt		$ 0	$ 500
Long-term Debt, Weighted Average Interest Rate		0.00%	2.50%
Union Bank, N.A. and other subsidiaries | Senior debt | Fixed rate 3.00% notes due June 2016
Debt Instrument
Long-term Debt		$ 700	$ 700
Debt instrument, Fixed interest rate (as a percent)		3.00%	3.00%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating rate notes due September 2016
Debt Instrument
Long-term Debt		$ 500	$ 500
Long-term Debt, Weighted Average Interest Rate		1.38%	1.35%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating rate notes due September 2016 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.75%	0.75%
Union Bank, N.A. and other subsidiaries | Senior debt | Fixed rate 2.125% notes due June 2017
Debt Instrument
Long-term Debt		$ 499	$ 499
Debt instrument, Fixed interest rate (as a percent)		2.125%	2.125%
Union Bank, N.A. and other subsidiaries | Senior debt | Fixed rate 2.625% notes due September 2018
Debt Instrument
Long-term Debt		$ 998	$ 1,000
Debt instrument, Fixed interest rate (as a percent)		2.625%	2.625%
Union Bank, N.A. and other subsidiaries | Senior debt | Fixed rate 1.50% notes due September 2016
Debt Instrument
Long-term Debt		$ 500	$ 499
Debt instrument, Fixed interest rate (as a percent)		1.50%	1.50%
Union Bank, N.A. and other subsidiaries | Senior debt | Fixed rate 2.250% notes due May 2019
Debt Instrument
Long-term Debt		$ 509	$ 502
Debt instrument, Fixed interest rate (as a percent)		2.25%	2.25%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating rate notes due May 2017
Debt Instrument
Long-term Debt		$ 250	$ 250
Long-term Debt, Weighted Average Interest Rate		1.02%	0.73%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating rate notes due May 2017 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.40%	0.40%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Debt Due January 2018 Issued August 2015 [Member]
Debt Instrument
Long-term Debt, Weighted Average Interest Rate		1.29%	1.09%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Debt Due January 2018 Issued August 2015 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.85%	0.85%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Debt Due January 2018 Issued September 2015 [Member]
Debt Instrument
Long-term Debt, Weighted Average Interest Rate		1.31%	1.11%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Debt Due January 2018 Issued September 2015 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		0.87%	0.87%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Debt Due January 2018 Issued October 2015 [Member]
Debt Instrument
Long-term Debt, Weighted Average Interest Rate		1.47%	1.27%
Union Bank, N.A. and other subsidiaries | Senior debt | Floating Rate Debt Due January 2018 Issued October 2015 [Member] | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		1.03%	1.03%
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Floating Rate Debt Due January 2018 Issued August 2015 [Member]
Debt Instrument
Long-term Debt		$ 1,000	$ 1,000
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Fixed rate 5.95% notes due May 2016
Debt Instrument
Long-term Debt		$ 701	$ 703
Debt instrument, Fixed interest rate (as a percent)		5.95%	5.95%
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Floating rate notes due to BTMU June 2023
Debt Instrument
Long-term Debt		$ 750	$ 750
Long-term Debt, Weighted Average Interest Rate		1.83%	1.80%
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Floating rate notes due to BTMU June 2023 | London Interbank Offered Rate LIBOR
Debt Instrument
Interest rate above variable interest rate (as a percent)		1.20%	1.20%
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Capital Lease Obligations
Debt Instrument
Long-term Debt, Weighted Average Interest Rate		4.92%	4.92%
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Floating Rate Debt Due January 2018 Issued September 2015 [Member]
Debt Instrument
Long-term Debt		$ 1,500	$ 1,500
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Floating Rate Debt Due January 2018 Issued October 2015 [Member]
Debt Instrument
Long-term Debt		1,000
Union Bank, N.A. and other subsidiaries | Subordinated Debt | Floating Rate Debt Due January 2018 Issued October 2015 [Member] [Member]
Debt Instrument
Long-term Debt			1,000
Union Bank, N.A. and other subsidiaries | Junior subordinated debt payable to trusts | Capital Lease Obligations
Debt Instrument
Long-term Debt	[2]	$ 14	$ 14

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[2] Long-term debt assumed through acquisitions.