Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Name:	us-gaap_IncomeLossFromEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Equity Method Investments - Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
MSMS
Equity Method Investments
Voting interest (as a percent)	40.00%		40.00%
MSMS \| MUFG
Equity Method Investments
Voting interest (as a percent)	60.00%		60.00%
MUMSS
Equity Method Investments
Income from equity method investments	$ (4)	$ 17	$ 5	$ 99
Voting interest (as a percent)	40.00%		40.00%
Institutional Securities \| MSMS
Equity Method Investments
Voting interest (as a percent)	51.00%		51.00%