Quarterly financial data (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly financial data (unaudited)
Product sales	$ 3,907	$ 3,877	$ 3,893	$ 3,618	$ 3,760	$ 3,759	$ 3,613	$ 3,528	$ 15,295	$ 14,660	$ 14,351
Gross profit from product sales	3,251	3,272	3,291	3,054	3,188	3,172	3,060	3,020
Net income	$ 934	$ 454	$ 1,170	$ 1,125	$ 1,022	$ 1,236	$ 1,202	$ 1,167	$ 3,683	$ 4,627	$ 4,605
Earnings per share:
Basic	$ 1.09	$ 0.50	$ 1.26	$ 1.21	$ 1.09	$ 1.29	$ 1.25	$ 1.19	$ 4.07	$ 4.82	$ 4.53
Diluted	$ 1.08	$ 0.50	$ 1.25	$ 1.20	$ 1.08	$ 1.28	$ 1.25	$ 1.18	$ 4.04	$ 4.79	$ 4.51