Accounting Standards Pension Accounting Standard (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Production	$ 842	$ 716	$ 720
Exploration	289	409	323
General and Administrative Expense	394	371	371
Operating Income (Loss)	1,667	(114)	(730)
Other net periodic benefit costs(a)	$ 14	19	102
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Production		706	712
Exploration		409	323
General and Administrative Expense		400	481
Operating Income (Loss)		(133)	(832)
Other net periodic benefit costs(a)		0	0
Accounting Standards Update 2017-07 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Production		10	8
Exploration		0	0
General and Administrative Expense		(29)	(110)
Operating Income (Loss)		19	102
Other net periodic benefit costs(a)		$ (19)	$ (102)