Investments - (Details) € in Millions, $ in Millions				3 Months Ended		12 Months Ended				36 Months Ended
	Feb. 21, 2018 USD ($)	Feb. 21, 2018 EUR (€)	Mar. 29, 2017 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Jun. 30, 2019	Oct. 03, 2018
Schedule of Equity Method Investments [Line Items]
Return of capital from equity method investment						$ 60	$ 0	$ 0
Options Clearing Corporation
Schedule of Equity Method Investments [Line Items]
Ownership percentage											40.00%
Equity capital					$ 150
Investment contribution					$ 60
Dividends received										$ 31
Return of capital from equity method investment				$ 16		60
Options Clearing Corporation | 2018 Net Income
Schedule of Equity Method Investments [Line Items]
Income from equity method investments						19
Options Clearing Corporation | 2019 Net Income
Schedule of Equity Method Investments [Line Items]
Income from equity method investments						43
Euroclear
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired	5.10%	5.10%						4.70%	4.70%	4.70%
Payments to acquire businesses, gross	$ 304	€ 246				$ 631		$ 327	€ 276
Total percentage of voting interests acquired				9.80%		9.80%
Dividend Income						$ 19	15
Cetip
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired			12.00%
Dividend Income								5
Realized investment gain			$ 176
Expenses and losses recognized			$ 9
The Clearing Corporation
Schedule of Equity Method Investments [Line Items]
Income from equity method investments						62	46	$ 36
MERSCORP Holdings, Inc
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired												100.00%
Required reserve				$ 42		42	$ 81
Investments sold						$ 41