Stock-based compensation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of stock-based compensation expense
Total stock-based compensation expense, pre-tax	$ 341	$ 353	$ 284
Tax benefit from stock-based compensation expense	(124)	(120)	(97)
Total stock-based compensation expense, net of tax	217	233	187
Stock Options [Member]
Components of stock-based compensation expense
Total stock-based compensation expense, pre-tax	85	124	115
Restricted Stock Units [Member]
Components of stock-based compensation expense
Total stock-based compensation expense, pre-tax	188	182	134
Performance Units [Member]
Components of stock-based compensation expense
Total stock-based compensation expense, pre-tax	$ 68	$ 47	$ 35