Segment Reporting (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues, less transaction-based expenses	$ 1,129	$ 797	$ 2,283	$ 1,647
Operating income	551	430	1,135	892
Total operating expenses	578	367	1,148	755
Net income	363	289	740	612	$ 1,295
Trading and Clearing Segment [Member]
Segment Reporting Information [Line Items]
Total revenues, less transaction-based expenses	527	491	1,101	1,040
Operating income	313	272	674	595
Total operating expenses	214	219	427	445
Data and Listings Segments [Member]
Segment Reporting Information [Line Items]
Total revenues, less transaction-based expenses	602	306	1,182	607
Operating income	238	158	461	297
Total operating expenses	$ 364	$ 148	$ 721	$ 310