Debt and lines of credit (Details) (USD $)	3 Months Ended				1 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Interest Rate Swap Cash Flow Hedges	Aug. 31, 2012 Notes Payable [Member]	May 31, 2011 Notes Payable [Member]	Mar. 31, 2013 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate)	Dec. 31, 2012 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate)	Mar. 31, 2013 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate) Interest Rate Swap Cash Flow Hedges	Mar. 31, 2013 Notes Payable [Member] Notes due 2013 at 0.875%	Dec. 31, 2012 Notes Payable [Member] Notes due 2013 at 0.875%	Mar. 31, 2013 Notes Payable [Member] Notes due 2014 at 1.375%	Dec. 31, 2012 Notes Payable [Member] Notes due 2014 at 1.375%	Mar. 31, 2013 Notes Payable [Member] Notes due 2015 at 0.45%	Dec. 31, 2012 Notes Payable [Member] Notes due 2015 at 0.45%	Aug. 31, 2012 Notes Payable [Member] Notes due 2015 at 0.45%	Mar. 31, 2013 Notes Payable [Member] Notes due 2016 at 2.375%	Dec. 31, 2012 Notes Payable [Member] Notes due 2016 at 2.375%	Mar. 31, 2013 Notes Payable [Member] Notes due 2019 at 1.65%	Dec. 31, 2012 Notes Payable [Member] Notes due 2019 at 1.65%	Aug. 31, 2012 Notes Payable [Member] Notes due 2019 at 1.65%	Mar. 31, 2013 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Dec. 31, 2012 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Mar. 31, 2013 National [Member] Notes Payable [Member] Notes due 2017 at 6.60% (assumed with National acquisition)	Dec. 31, 2012 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	0
Long-term Debt
Long-term Debt, Gross	5,625,000,000		5,625,000,000		1,500,000,000		1,000,000,000	1,000,000,000		500,000,000	500,000,000	1,000,000,000	1,000,000,000	750,000,000	750,000,000	750,000,000	1,000,000,000	1,000,000,000	750,000,000	750,000,000	750,000,000	250,000,000	250,000,000	375,000,000	375,000,000
Add net unamortized premium	58,000,000		61,000,000
Less current portion of long-term debt	(1,500,000,000)		(1,500,000,000)
Total long-term debt	4,183,000,000		4,186,000,000
Long-term debt, stated interest rate							0.922%			0.875%		1.375%		0.45%			2.375%		1.65%			3.95%		6.60%
Long-term debt, effective fixed-rate interest rate									0.922%
Payments of Debt Issuance Costs					7,000,000	12,000,000
Proceeds from Issuance of Long-term Debt					1,492,000,000	3,497,000,000
Interest Rate Cash Flow Hedge Liability at Fair Value				1,000,000
Interest and debt expense	$ 23,000,000	$ 21,000,000