Investments - Narrative (Details) - Options Clearing Corporation - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Schedule of Equity Method Investments [Line Items]
Investment ownership percentage	40.00%
2019 Net Income
Schedule of Equity Method Investments [Line Items]
Equity earnings in other income	$ 17	$ 27
2018 Net Income
Schedule of Equity Method Investments [Line Items]
Equity earnings in other income		$ 19

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

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