Supplemental Cash Flow Information (Change in Cash and Cash Equivalents) (Details) - USD ($) $ in Millions		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents		$ 1,687		$ 3,011
Restricted cash	[1]	38		4
Cash, cash equivalents and restricted cash		$ 1,725	[2]	$ 3,015	[2]	$ 892	$ 1,136

[1] The restricted cash balance is included within other current assets on the consolidated balance sheets.

[2] As a result of the adoption of ASU 2016-18, the consolidated statements of cash flows now explain the change during the period of both cash and cash equivalents and restricted cash.