Debt and lines of credit (Details) (USD $)	3 Months Ended				1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 09, 2012	Dec. 31, 2011	May 31, 2011 Notes Payable [Member]	Mar. 31, 2012 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate)	Dec. 31, 2011 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate)	Mar. 31, 2012 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate) Interest Rate Swap [Member] Cash Flow Hedges [Member]	Mar. 31, 2012 Notes Payable [Member] Notes due 2013 at 0.875%	Dec. 31, 2011 Notes Payable [Member] Notes due 2013 at 0.875%	Mar. 31, 2012 Notes Payable [Member] Notes due 2014 at 1.375%	Dec. 31, 2011 Notes Payable [Member] Notes due 2014 at 1.375%	Mar. 31, 2012 Notes Payable [Member] Notes due 2016 at 2.375%	Dec. 31, 2011 Notes Payable [Member] Notes due 2016 at 2.375%	Mar. 31, 2012 National [Member] Notes Payable [Member]	Mar. 31, 2012 National [Member] Notes Payable [Member] Notes due 2012 at 6.15% (assumed with National acquisition)	Dec. 31, 2011 National [Member] Notes Payable [Member] Notes due 2012 at 6.15% (assumed with National acquisition)	Mar. 31, 2012 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Dec. 31, 2011 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Mar. 31, 2012 National [Member] Notes Payable [Member] Notes due 2017 at 6.60% (assumed with National acquisition)	Dec. 31, 2011 National [Member] Notes Payable [Member] Notes due 2017 at 6.60% (assumed with National acquisition)
Debt Instrument
Line of credit, term (in years)	5 years
Line of credit, borrowing capacity			$ 2,000,000,000
Short-term Debt
Commercial paper outstanding	700,000,000			999,000,000
Debt, weighted average interest rate (as a percent)	0.30%
Long-term Debt
Long-term Debt, Gross	4,500,000,000			4,500,000,000		1,000,000,000	1,000,000,000		500,000,000	500,000,000	1,000,000,000	1,000,000,000	1,000,000,000	1,000,000,000	1,000,000,000	375,000,000	375,000,000	250,000,000	250,000,000	375,000,000	375,000,000
Add net unamortized premium (assumed with National acquisition)	85,000,000			93,000,000
Less current portion of long-term debt	(378,000,000)			(382,000,000)
Total long-term debt	4,207,000,000			4,211,000,000
Long-term debt, stated interest rate						0.922%			0.875%		1.375%		2.375%			6.15%		3.95%		6.60%
Long-term debt, effective fixed-rate interest rate								0.922%
Payments of Debt Issuance Costs					12,000,000
Proceeds from Issuance of Long-term Debt					3,497,000,000
Long-term Debt, Fair Value															1,105,000,000
Interest and debt expense	$ 21,000,000	$ 0