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		$ 453					453
Foreign currency translation adjustments		(46)	[3]						(46)	[2]
Fixed price diesel swaps	[2]								1
Stock compensation expense, net (in shares)	[1]			1
Stock compensation expense, net						43
Exercise of common stock options						2
Shares repurchased and retired						(50)
Repurchase of common stock (in shares)				(2)	[1]			2
Repurchase of common stock								$ (345)
Balance (in shares) at Jun. 30, 2018				83	[1]			30
Balance at Jun. 30, 2018				$ 1		2,351	3,458	$ (2,450)	(196)	[2]
Balance (in shares) at Mar. 31, 2018				84	[1]			29
Balance at Mar. 31, 2018				$ 1		2,327	3,188	$ (2,282)	(176)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		270					270
Foreign currency translation adjustments		(21)	[3]						(21)	[2]
Fixed price diesel swaps	[2]								1
Stock compensation expense, net (in shares)	[1]			0
Stock compensation expense, net						24
Exercise of common stock options						1
Shares repurchased and retired						(1)
Repurchase of common stock (in shares)				(1)	[1]			1
Repurchase of common stock								$ (168)
Balance (in shares) at Jun. 30, 2018				83	[1]			30
Balance at Jun. 30, 2018				$ 1		2,351	3,458	$ (2,450)	(196)	[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		445					445
Foreign currency translation adjustments		42	[3]						42	[2]
Fixed price diesel swaps	[2]								1
Stock compensation expense, net (in shares)	[1]			0
Stock compensation expense, net						31
Exercise of common stock options						10
Shares repurchased and retired						(34)
Repurchase of common stock (in shares)				(3)	[1]			3
Repurchase of common stock								$ (420)
Balance (in shares) at Jun. 30, 2019				77	[1]			36
Balance at Jun. 30, 2019		3,478		$ 1		2,415	4,546	$ (3,290)	(194)	[2]
Balance (in shares) at Mar. 31, 2019				79	[1]			35
Balance at Mar. 31, 2019				$ 1		2,394	4,276	$ (3,080)	(216)	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		270					270
Foreign currency translation adjustments		22	[3]						22	[2]
Stock compensation expense, net (in shares)	[1]			(1)
Stock compensation expense, net						16
Exercise of common stock options						6
Shares repurchased and retired						(1)
Repurchase of common stock (in shares)				(1)	[1]			1
Repurchase of common stock								$ (210)
Balance (in shares) at Jun. 30, 2019				77	[1]			36
Balance at Jun. 30, 2019		$ 3,478		$ 1		$ 2,415	$ 4,546	$ (3,290)	$ (194)	[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.