Profit per share (Details) - USD ($) $ / shares in Units, $ 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	Sep. 30, 2014	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014		Jan. 31, 2014	Feb. 28, 2007
Earnings Per Share [Abstract]
Profit for the period (A) (in millions of dollars)		$ (1,171)	$ 283	$ 550	$ 271	$ (94)	$ 559	$ 802	$ 1,245		$ (67)	[1]	$ 2,512	[1]	$ 2,452	[1]
Determination of shares (in millions)
Weighted-average number of common shares outstanding (B) (in shares)											584,300,000		594,300,000		617,200,000
Shares issuable on exercise of stock awards, net of shares assumed to be purchased out of proceeds at average market price (in shares)											0		7,000,000		11,700,000
Average common shares outstanding for fully diluted computation (C) (in shares)	[2]										584,300,000		601,300,000		628,900,000
Profit (loss) per share of common stock:
Assuming no dilution (A/B) (in dollars per share)		$ (2.00)	$ 0.48	$ 0.94	$ 0.46	$ (0.16)	$ 0.95	$ 1.33	$ 2.06		$ (0.11)		$ 4.23		$ 3.97
Assuming full dilution (A/C) (in dollars per share)		$ (2.00)	$ 0.48	$ 0.93	$ 0.46	$ (0.16)	$ 0.94	$ 1.31	$ 2.03		$ (0.11)	[2]	$ 4.18	[2]	$ 3.90	[2]
Shares outstanding as of December 31		586,500,000				582,300,000					586,500,000		582,300,000		606,200,000
Common shares under SARs and stock options not included in the computation of diluted earnings per share (in shares)											32,064,790		22,169,133		10,266,682
Common Stock Repurchase
Stock Repurchase Program, Authorized Amount																	$ 10,000	$ 7,500
Common shares repurchased (in shares)							19,600,000		18,100,000	23,700,000	0		25,841,608		41,762,325
Payments for repurchase of common stock							$ 1,500		$ 1,700	$ 2,500	$ 0		$ 2,025		$ 4,238
Stock repurchase program, amount of authorized repurchase spent to date											$ 4,500

[1] 1 Profit (loss) attributable to common stockholders.

[2] 2 Diluted by assumed exercise of stock-based compensation awards, using the treasury stock method.