Debt and lines of credit (Details) (USD $)	3 Months Ended		6 Months Ended			1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	May 31, 2013 Notes Payable [Member]	Aug. 31, 2012 Notes Payable [Member]	Jun. 30, 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)	Jun. 30, 2013 Notes Payable [Member] Notes due 2013 at 0.875%	Dec. 31, 2012 Notes Payable [Member] Notes due 2013 at 0.875%	Jun. 30, 2013 Notes Payable [Member] Notes due 2014 at 1.375%	Dec. 31, 2012 Notes Payable [Member] Notes due 2014 at 1.375%	Jun. 30, 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%	Jun. 30, 2013 Notes Payable [Member] Notes due 2016 at 2.375%	Dec. 31, 2012 Notes Payable [Member] Notes due 2016 at 2.375%	Jun. 30, 2013 Notes Payable [Member] Notes due 2018 at 1.00%	May 31, 2013 Notes Payable [Member] Notes due 2018 at 1.00%	Dec. 31, 2012 Notes Payable [Member] Notes due 2018 at 1.00%	Jun. 30, 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%	Jun. 30, 2013 Notes Payable [Member] Notes due 2023 at 2.25%	May 31, 2013 Notes Payable [Member] Notes due 2023 at 2.25%	Dec. 31, 2012 Notes Payable [Member] Notes due 2023 at 2.25%	Jun. 30, 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)	Jun. 30, 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, borrowing capacity	$ 2,000,000,000		$ 2,000,000,000
Short-term Debt
Commercial paper	0		0
Long-term Debt
Long-term debt, stated interest rate								0.922%		0.875%		1.375%		0.45%			2.375%		1.00%			1.65%			2.25%			3.95%		6.60%
Long-term Debt, Gross	5,125,000,000		5,125,000,000		5,625,000,000	1,000,000,000	1,500,000,000	0	1,000,000,000	0	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	500,000,000	500,000,000	0	750,000,000	750,000,000	750,000,000	500,000,000	500,000,000	0	250,000,000	250,000,000	375,000,000	375,000,000
Add net unamortized premium	40,000,000		40,000,000		61,000,000
Less current portion of long-term debt	(1,000,000,000)		(1,000,000,000)		(1,500,000,000)
Total long-term debt	4,165,000,000		4,165,000,000		4,186,000,000
Proceeds from Issuance of Long-term Debt						986,000,000	1,492,000,000
Repayments of Long-term Debt						1,500,000,000
Payments of Debt Issuance Costs						7,000,000	7,000,000
Interest and debt expense	$ 24,000,000	$ 20,000,000	$ 47,000,000	$ 41,000,000