New Accounting Standard (Unaudited) Pension/Postretirement ASU (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of Goods Sold, Excluding Depreciation, Depletion, and Amortization	$ 3,069	$ 2,794	$ 8,792	$ 7,462
Cost of Revenue	3,527	3,212	10,143	8,719
Selling, general and administrative expenses	101	104	341	362
Mining exploration and research expenses	27	27	72	60
Environmental obligations and shutdown costs	8	72	76	76
Costs and Expenses	3,593	3,382	10,506	9,151
Operating income (loss)	1,315	928	4,438	2,211
Other Nonoperating Income (Expense)	$ 14	(9)	$ 63	(9)
Restatement Adjustment [Member] | Accounting Standards Update 2017-05 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of Goods Sold, Excluding Depreciation, Depletion, and Amortization		(8)		(35)
Cost of Revenue		(8)		(35)
Selling, general and administrative expenses		(2)		(4)
Mining exploration and research expenses				(1)
Environmental obligations and shutdown costs		(1)		(5)
Costs and Expenses		(11)		(45)
Operating income (loss)		11		45
Other Nonoperating Income (Expense)		(11)		(45)
Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of Goods Sold, Excluding Depreciation, Depletion, and Amortization		2,802		7,497
Cost of Revenue		3,220		8,754
Selling, general and administrative expenses		106		366
Mining exploration and research expenses				61
Environmental obligations and shutdown costs		73		81
Costs and Expenses		3,393		9,196
Operating income (loss)		917		2,166
Other Nonoperating Income (Expense)		$ 2		$ 36