Recently Adopted and Recently Issued Accounting Guidance (Details) - USD ($) $ in Millions	Jan. 01, 2019	Mar. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets classified in Other noncurrent assets		$ 302
Operating lease liability		$ 309
Adoption of accounting standards (B)	$ 73
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Right-of-use assets classified in Other noncurrent assets	320
Operating lease liability	320
Accounting Standards Update 2017-12
Summary Of Significant Accounting Policies [Line Items]
Adoption of accounting standards (B)	2
Other Noncurrent Liabilities and Deferred Credits | Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Effect on retained earnings	119
Property, Plant and Equipment | Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Effect on retained earnings	(24)
Other Noncurrent Assets | Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Effect on retained earnings	$ (22)