Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		36 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2015	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment contribution				$ 0	$ 305
Return of capital from equity method investment				$ 44	$ 0
Options Clearing Corporation
Schedule of Equity Method Investments [Line Items]
Investment ownership percentage		40.00%		40.00%
Equity capital			$ 150
Investment contribution			$ 60
Dividends received						$ 31
Return of capital from equity method investment				$ 44
Scenario, Forecast | Options Clearing Corporation
Schedule of Equity Method Investments [Line Items]
Return of capital from equity method investment	$ 16
2018 Net Income | Options Clearing Corporation
Schedule of Equity Method Investments [Line Items]
Equity earnings in other income				19
2019 Net Income | Options Clearing Corporation
Schedule of Equity Method Investments [Line Items]
Equity earnings in other income		$ 15		$ 32