CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) shares in Millions, $ in Millions		Total		Common Stock		Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss
Balance (in shares) at Dec. 31, 2017				84	[1]			28
Balance at Dec. 31, 2017				$ 1		$ 2,356	$ 3,005	$ (2,105)	$ (151)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		$ 786					786
Foreign currency translation adjustments		(28)	[3]						(28)	[2]
Fixed price diesel swaps	[2]								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 (in shares) at Sep. 30, 2018				82	[1]			31
Balance at Sep. 30, 2018				$ 1		2,380	3,791	$ (2,660)	(178)	[2]
Balance (in shares) at Jun. 30, 2018				83	[1]			30
Balance at Jun. 30, 2018				$ 1		2,351	3,458	$ (2,450)	(196)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		333					333
Foreign currency translation adjustments		18	[3]						18	[2]
Stock compensation expense, net (in shares)	[1]			0
Stock compensation expense, net						30
Shares repurchased and retired						(1)
Repurchase of common stock (in shares)				(1)	[1]			(1)
Repurchase of common stock								$ (210)
Balance (in shares) at Sep. 30, 2018				82	[1]			31
Balance at Sep. 30, 2018				$ 1		2,380	3,791	$ (2,660)	(178)	[2]
Balance (in shares) at Dec. 31, 2018				80	[1]			33
Balance at Dec. 31, 2018		3,403		$ 1		2,408	4,101	$ (2,870)	(237)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		836					836
Foreign currency translation adjustments		19	[3]						19	[2]
Fixed price diesel swaps	[2]								1
Stock compensation expense, net (in shares)	[1]			1
Stock compensation expense, net						45
Exercise of common stock options						10
Shares repurchased and retired						(34)
Repurchase of common stock (in shares)				(5)	[1]			(5)
Repurchase of common stock								$ (630)
Balance (in shares) at Sep. 30, 2019				76	[1]			38
Balance at Sep. 30, 2019		3,650		$ 1		2,429	4,937	$ (3,500)	(217)	[2]
Balance (in shares) at Jun. 30, 2019				77	[1]			36
Balance at Jun. 30, 2019				$ 1		2,415	4,546	$ (3,290)	(194)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		391					391
Foreign currency translation adjustments		(23)	[3]						(23)	[2]
Stock compensation expense, net (in shares)	[1]			1
Stock compensation expense, net						14
Repurchase of common stock (in shares)				(2)	[1]			(2)
Repurchase of common stock								$ (210)
Balance (in shares) at Sep. 30, 2019				76	[1]			38
Balance at Sep. 30, 2019		$ 3,650		$ 1		$ 2,429	$ 4,937	$ (3,500)	$ (217)	[2]

[1] Common stock outstanding decreased by less than 5 million net shares during the year ended December 31, 2018.

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

[3] There were no material reclassifications from accumulated other comprehensive loss reflected in other comprehensive income (loss) during 2019 or 2018. There is no tax impact related to the foreign currency translation adjustments, as the earnings are considered permanently reinvested. We have historically considered the undistributed earnings of our foreign subsidiaries to be indefinitely reinvested. We have not repatriated funds to the U.S. to satisfy domestic liquidity needs, nor do we anticipate the need to do so. If we determine that all or a portion of our foreign earnings are no longer indefinitely reinvested, we may be subject to additional foreign withholding taxes and U.S. state income taxes. There were no material taxes associated with other comprehensive income (loss) during 2019 or 2018.