Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities:
Net Income	$ 150	$ (751)	$ 1,049
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Other, net	(7)	59	(18)
Cash Flows from Financing Activities:
Proceeds from (Repurchase of) Equity	0	0	(2,496)
Other, net	(59)	(96)	(115)
Cash and cash equivalents	9,641	8,350
Cash, Cash Equivalents, and Federal Funds Sold	16,414	9,641	8,350
MUFG Americas Holdings Corporation
Cash Flows from Operating Activities:
Net Income	166	(734)	1,073
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Equity in undistributed net income of subsidiaries less dividends received	(147)	1,270	676
Other, net	58	(743)	(9)
Net cash provided by (used in) operating activities	77	(207)	1,740
Cash Flows from Investing Activities:
Investments in and advances to subsidiaries	(564)	(1,585)	(2,612)
Repayment of investments in and advances to subsidiaries	584	1,634	2,411
Net cash provided by (used in) investing activities	20	49	(201)
Cash Flows from Financing Activities:
Proceeds from advances from subsidiaries	0	0	0
Repayment of advances from subsidiaries	0	0	0
Proceeds from issuance of long-term debt	3,025	1,890	6,500
Repayment of long-term debt	(3,379)	(1,631)	(6,145)
Dividends paid	0	0	0
Proceeds from (Repurchase of) Equity	0	0	(2,496)
Net cash provided by (used in) financing activities	(354)	259	(2,141)
Net increase (decrease) in cash and cash equivalents	(257)	101	(602)
Cash and cash equivalents	840	739	1,341
Cash, Cash Equivalents, and Federal Funds Sold	$ 583	$ 840	$ 739