Segment Information (details) - Enterprise-Wide Disclosures - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues based on location
Benchmark percentage of revenue transactions from a single customer									10.00%	10.00%	10.00%
Total revenues	$ 7,451	$ 7,325	$ 7,184	$ 6,942	$ 7,193	$ 6,961	$ 6,785	$ 6,686	$ 28,902	$ 27,625	$ 26,815
U.S. [Member]
Revenues based on location
Total revenues									27,253	25,904	25,127
Non-U.S. [Member]
Revenues based on location
Total revenues									1,649	1,721	1,688
Canada [Member]
Revenues based on location
Total revenues									1,232	1,154	1,202
Other Non-U.S. [Member]
Revenues based on location
Total revenues									$ 417	$ 567	$ 486