Stock-Based Compensation Plans - Balance Sheet Classification of Obligations related to Outstanding Performance Share Awards Not Yet Settled (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Classification Of Obligations Related To Outstanding Performance Share Awards Not Yet Settled Current Liabilities	[1]	$ 57	$ 49
Classification Of Obligations Related To Outstanding Performance Share Awards Not Yet Settled Deferred Credits And Other Liabilities	[2]	100	52
Classification Of Obligations Related To Outstanding Performance Share Awards Not Yet Settled Common Stock		26	40
Total Balance Sheet Classification Of Obligations Related To Outstanding Performance Share Awards Not Yet Settled		$ 183	$ 141

[1] Represents the current liability related to performance share awards expected to be settled in cash.

[2] Represents the long-term liability related to performance share awards expected to be settled in cash.