CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY (UNAUDITED) - 9 months ended Sep. 30, 2018 - USD ($) shares in Millions, $ in Millions		Total	Common Stock		Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss	[2]
Balance (shares) at Dec. 31, 2017			84	[1]			(28)
Balance at Dec. 31, 2017		$ 3,106	$ 1		$ 2,356	$ 3,005	$ (2,105)	$ (151)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		786				786
Foreign currency translation adjustments		(28)						(28)
Fixed price diesel swaps		1						1
Stock compensation expense, net (in shares)	[1]		1
Stock compensation expense, net					73
Exercise of common stock options					2
Shares repurchased and retired					(51)
Repurchase of common stock (in shares)			(3)	[1]			3
Repurchase of common stock							$ (555)
Balance (shares) at Sep. 30, 2018			82	[1]			(31)
Balance at Sep. 30, 2018		$ 3,334	$ 1		$ 2,380	$ 3,791	$ (2,660)	$ (178)

[1] Common stock outstanding increased by less than 1 million net shares during the year ended December 31, 2017.

[2] The Accumulated Other Comprehensive Loss balance primarily reflects foreign currency translation adjustments.