Business Segments (Schedule of Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 1,414	$ 1,096	$ 4,165	$ 3,197
Transaction-based expenses	(699)	(464)	(2,050)	(1,308)
Revenues less transaction-based expenses	715	632	2,115	1,889
Operating income	326	226	916	757
Operating Segments | Market Services
Segment Reporting Information [Line Items]
Total revenues	958	690	2,867	1,995
Transaction-based expenses	(699)	(464)	(2,050)	(1,308)
Revenues less transaction-based expenses	259	226	817	687
Operating income	154	129	509	392
Operating Segments | Corporate Services
Segment Reporting Information [Line Items]
Total revenues	132	124	386	368
Transaction-based expenses	0	0	0	0
Revenues less transaction-based expenses	132	124	386	368
Operating income	51	45	145	134
Operating Segments | Information Services
Segment Reporting Information [Line Items]
Total revenues	238	198	661	585
Transaction-based expenses	0	0	0	0
Revenues less transaction-based expenses	238	198	661	585
Operating income	155	126	420	372
Operating Segments | Market Technology
Segment Reporting Information [Line Items]
Total revenues	86	84	251	239
Transaction-based expenses	0	0	0	0
Revenues less transaction-based expenses	86	84	251	239
Operating income	9	15	32	29
Corporate Items
Segment Reporting Information [Line Items]
Total revenues	0	0	0	10
Transaction-based expenses	0	0	0	0
Revenues less transaction-based expenses	0	0	0	10
Operating income	$ (43)	$ (89)	$ (190)	$ (170)