Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per share:
Earnings available to common stockholders									$ 2,263	$ 7,340	$ 1,132
Basic weighted-average shares outstanding									329.9	376.1	371.1
Earnings per share, basic	$ 1.26	$ 3.02	$ 1.78	$ 0.88	$ 2.24	$ 12.83	$ 3.14	$ 1.33	$ 6.86	$ 19.52	$ 3.05
Diluted earnings per share:
Earnings available to common stockholders									$ 2,263	$ 7,340	$ 1,132
Effect of dilutive securities											11
Earnings available to common stockholders including the effect of dilutive securities									$ 2,263	$ 7,340	$ 1,143
Diluted shares outstanding:
Basic weighted-average shares outstanding									329.9	376.1	371.1
Effect of convertible notes										0.3	17.7
Effect of employee stock awards									0.4	0.5	0.9
Diluted weighted-average shares outstanding									330.3	376.9	389.7
Earnings per share, diluted	$ 1.26	$ 3.01	$ 1.78	$ 0.88	$ 2.24	$ 12.82	$ 3.14	$ 1.32	$ 6.85	$ 19.47	$ 2.93