Business Segments (Corporate Services and Market Technology Segments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total revenues									$ 3,705	$ 3,403	$ 3,500
Operating Income									839	720	754
Operating Segments \| Corporate Services
Segment Reporting Information [Line Items]
Total revenues	$ 167	$ 162	$ 162	$ 143	$ 143	$ 138	$ 142	$ 139	635	562	552
Operating Income	41	42	41	34	36	36	37	32	158	140	121
Operating Segments \| Market Technology
Segment Reporting Information [Line Items]
Total revenues	77	73	69	57	71	59	59	55	275	245	246
Operating Income	$ 23	$ 19	$ 17	$ 10	$ 23	$ 12	$ 11	$ 10	$ 69	$ 58	$ 49