BUSINESS SEGMENT AND FOREIGN INFORMATION - Yen/Dollar Exchange Rates Used (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2020 USD ($) ¥ / $	Dec. 31, 2019 USD ($) ¥ / $	Dec. 31, 2018 USD ($) ¥ / $
Segment Reporting, Other Significant Reconciling Item [Line Items]
Weighted-average yen/dollar exchange rate | ¥ / $	[1]	106.86	109.07	110.39
Yen percent strengthening (weakening)		2.10%	1.20%	1.60%
Exchange effect on pretax operating earnings (in millions)		$ 38	$ 20	$ 38
Yen/dollar exchange rate at December 31 | ¥ / $	[1]	103.50	109.56
Yen percent strengthening (weakening)		5.90%	1.30%
Exchange effect on total assets
Segment Reporting, Other Significant Reconciling Item [Line Items]
Exchange effect		$ 7,970	$ 1,225
Exchange effect on total liabilities
Segment Reporting, Other Significant Reconciling Item [Line Items]
Exchange effect		$ 7,870	$ 1,533

[1] Rates are based on the published MUFG Bank, Ltd. telegraphic transfer middle rate (TTM)