Earnings (Loss) per Share - Computation of Basic and Diluted Earnings (Loss) per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic Earnings (Loss) Per Share:
Net income (loss)	$ 3,281	$ 1,693	$ 1,704	$ 932	$ 597	$ 942	$ 864	$ 480	$ 7,610	$ 2,882	$ (1,834)
Weighted-average shares outstanding (in thousands)	626,559	661,869	688,727	696,415	706,185	719,067	720,600	723,971	668,393	717,456	280,213
Basic earnings (loss) per share (in dollars per share)	$ 5.24	$ 2.56	$ 2.47	$ 1.34	$ 0.84	$ 1.31	$ 1.20	$ 0.66	$ 11.39	$ 4.02	$ (6.54)
Diluted Earnings (Loss) Per Share:
Net income (loss)	$ 3,281	$ 1,693	$ 1,704	$ 932	$ 597	$ 942	$ 864	$ 480	$ 7,610	$ 2,882	$ (1,834)
Change in fair value of conversion feature on 7.25% convertible senior notes										3
Net income (loss) for purposes of computing diluted earnings (loss) per share									$ 7,610	$ 2,885	$ (1,834)
Share computation for diluted earnings (loss) per share (in thousands):
Weighted-average shares outstanding	626,559	661,869	688,727	696,415	706,185	719,067	720,600	723,971	668,393	717,456	280,213
Dilutive effect of stock awards									18,962	15,603
Assumed conversion of convertible senior notes										957
Weighted average common shares outstanding - as adjusted	644,140	680,739	707,611	716,930	724,767	735,196	734,767	741,335	687,355	734,016	280,213
Diluted earnings (loss) per share (in dollars per share)	$ 5.09	$ 2.49	$ 2.41	$ 1.30	$ 0.82	$ 1.28	$ 1.17	$ 0.65	$ 11.07	$ 3.93	$ (6.54)
Stock options, SARs and RSUs because inclusion would be antidilutive									764	226	7,674