Summary of Significant Accounting Policies (Revenue by Product, Service and Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	$ 2,526	$ 2,411	$ 2,276
Transaction-based trading and clearing, net
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	666	590	561
Trade management services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	292	291	266
Corporate solutions
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	238	234	208
Listing services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	290	267	269
Market data products
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	390	369	354
Index
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	206	171	149
Investment data & analytics
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	118	48	37
Market technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	270	247	241
Other revenues
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	56	194	191
Operating Segments | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	958	881	827
Operating Segments | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	528	501	477
Operating Segments | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	714	588	540
Operating Segments | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	270	247	241
Operating Segments | Transaction-based trading and clearing, net | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	666	590	561
Operating Segments | Transaction-based trading and clearing, net | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Transaction-based trading and clearing, net | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Transaction-based trading and clearing, net | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Trade management services | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	292	291	266
Operating Segments | Trade management services | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Trade management services | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Trade management services | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Corporate solutions | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Corporate solutions | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	238	234	208
Operating Segments | Corporate solutions | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Corporate solutions | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Listing services | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Listing services | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	290	267	269
Operating Segments | Listing services | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Listing services | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market data products | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market data products | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market data products | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	390	369	354
Operating Segments | Market data products | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Index | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Index | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Index | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	206	171	149
Operating Segments | Index | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Investment data & analytics | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Investment data & analytics | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Investment data & analytics | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	118	48	37
Operating Segments | Investment data & analytics | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market technology | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market technology | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market technology | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Market technology | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	270	247	241
Operating Segments | Other revenues | Market Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Other revenues | Corporate Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Other revenues | Information Services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Operating Segments | Other revenues | Market Technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	56	194	191
Segment Reconciling Items | Transaction-based trading and clearing, net
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Trade management services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Corporate solutions
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Listing services
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Market data products
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Index
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Investment data & analytics
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Market technology
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	0	0	0
Segment Reconciling Items | Other revenues
Disaggregation of Revenue [Line Items]
Revenues less transaction-based expenses	$ 56	$ 194	$ 191