New Accounting Standard (Unaudited) Pension/Postretirement ASU (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Mineral Extraction Processing and Marketing Costs	$ 2,808	$ 2,188
Cost of Revenue	3,259	2,577
Selling, general and administrative expenses	131	151
Mining exploration and research expenses	21	14
Environmental obligations and shutdown costs	9	25
Costs and Expenses	3,409	2,744
Operating income (loss)	1,459	597
Other Nonoperating Income (Expense)	$ 28	8
Scenario, Previously Reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Mineral Extraction Processing and Marketing Costs		2,200
Cost of Revenue		2,589
Selling, general and administrative expenses		153
Mining exploration and research expenses		15
Environmental obligations and shutdown costs		27
Costs and Expenses		2,761
Operating income (loss)		580
Other Nonoperating Income (Expense)		25
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		(12)
Cost of Revenue		(12)
Selling, general and administrative expenses		(2)
Mining exploration and research expenses		(1)
Environmental obligations and shutdown costs		(2)
Costs and Expenses		(17)
Operating income (loss)		17
Other Nonoperating Income (Expense)		$ (17)