New Accounting Standard (Unaudited) Pension/Postretirement ASU (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Mineral Extraction Processing and Marketing Costs	$ 2,915	$ 2,480	$ 5,723	$ 4,668
Cost of Revenue	3,357	2,930	6,616	5,507
Selling, general and administrative expenses	109	107	240	258
Mining exploration and research expenses	24	19	45	33
Environmental obligations and shutdown costs	59	(21)	68	4
Costs and Expenses	3,504	3,025	6,913	5,769
Operating income (loss)	1,664	686	3,123	1,283
Other Nonoperating Income (Expense)	$ 20	(7)	$ 49	0
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Mineral Extraction Processing and Marketing Costs		2,495		4,695
Cost of Revenue		2,945		5,534
Selling, general and administrative expenses				260
Mining exploration and research expenses				34
Environmental obligations and shutdown costs		(19)		8
Costs and Expenses		3,042		5,803
Operating income (loss)		669		1,249
Other Nonoperating Income (Expense)		10		34
Accounting Standards Update 2017-05 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Mineral Extraction Processing and Marketing Costs		(15)		(27)
Cost of Revenue		(15)		(27)
Selling, general and administrative expenses				(2)
Mining exploration and research expenses				(1)
Environmental obligations and shutdown costs		(2)		(4)
Costs and Expenses		(17)		(34)
Operating income (loss)		17		34
Other Nonoperating Income (Expense)		$ (17)		$ (34)