Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended	36 Months Ended
	Jun. 30, 2019	Mar. 31, 2015	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2019	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	$ 9		$ 17