Segment Information - Total Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets
Total assets:	$ 10,281	$ 9,756	$ 9,459
Reportable Segment Totals
Assets
Total assets:	10,080	9,545	9,289
Americas
Assets
Total assets:	5,637	5,411	5,433
Europe
Assets
Total assets:	3,387	3,133	2,858
Asia Pacific
Assets
Total assets:	1,056	1,001	998
Other
Assets
Total assets:	$ 201	$ 211	$ 170