Stockholders' Equity - Earnings Per Share Reconciliation (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income (loss)
Net income (loss) attributable to common stockholders	$ 363	$ (699)	$ 513	$ (1,432)
Income (loss) effect of TEUs	0	(2)	(4)	(6)
Less distributions on participating securities	2	0	4	0
Less undistributed income allocated to participating securities	2	0	1	0
Basic	359	(701)	504	(1,438)
Income (loss) effect of TEUs	0	0	0	(1)
Diluted	$ 359	$ (701)	$ 504	$ (1,439)
Shares
Average number of common shares outstanding—basic	499	553	507	552
Dilutive effect of stock options	1	0	1	0
Average number of common shares outstanding—diluted	500	553	508	552
Excluded due to anti-dilutive effect	8	11	9	11
Net income (loss) per common share
Basic (in usd per share)	$ 0.72	$ (1.27)	$ 0.99	$ (2.60)
Diluted (in usd per share)	$ 0.72	$ (1.27)	$ 0.99	$ (2.61)