Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Assets	$ 17,064	$ 16,511	$ 16,391
Operating Segments
Segment Reporting Information [Line Items]
Assets	14,599	13,601	13,211
Operating Segments | Americas
Segment Reporting Information [Line Items]
Assets	6,923	6,701	6,275
Operating Segments | Europe, Middle East and Africa
Segment Reporting Information [Line Items]
Assets	4,995	4,385	4,377
Operating Segments | Asia Pacific
Segment Reporting Information [Line Items]
Assets	2,681	2,515	2,559
Corporate
Segment Reporting Information [Line Items]
Assets	$ 2,465	$ 2,910	$ 3,180