Business Segments (Schedule of Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 1,353	$ 1,039
Transaction-based expenses	(652)	(405)
Revenues less transaction-based expenses	701	634
Depreciation and amortization	48	48
Operating income	275	275
Operating Segments | Market Services
Segment Reporting Information [Line Items]
Total revenues	933	638
Transaction-based expenses	(652)	(405)
Revenues less transaction-based expenses	281	233
Operating income	178	135
Operating Segments | Corporate Services
Segment Reporting Information [Line Items]
Total revenues	128	121
Transaction-based expenses	0	0
Revenues less transaction-based expenses	128	121
Operating income	45	44
Operating Segments | Information Services
Segment Reporting Information [Line Items]
Total revenues	211	193
Transaction-based expenses	0	0
Revenues less transaction-based expenses	211	193
Operating income	134	124
Operating Segments | Market Technology
Segment Reporting Information [Line Items]
Total revenues	81	77
Transaction-based expenses	0	0
Revenues less transaction-based expenses	81	77
Operating income	8	7
Corporate Items
Segment Reporting Information [Line Items]
Total revenues	0	10
Transaction-based expenses	0	0
Revenues less transaction-based expenses	0	10
Operating income	$ (90)	$ (35)