Debt and lines of credit (Details) (USD $)	3 Months Ended			1 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2014 Notes Payable [Member]	May 31, 2013 Notes Payable [Member]	Mar. 31, 2014 Notes Payable [Member] Notes due 2014 at 1.375%	Dec. 31, 2013 Notes Payable [Member] Notes due 2014 at 1.375%	Mar. 31, 2014 Notes Payable [Member] Notes due 2015 at 0.45%	Dec. 31, 2013 Notes Payable [Member] Notes due 2015 at 0.45%	Mar. 31, 2014 Notes Payable [Member] Notes due 2016 at 2.375%	Dec. 31, 2013 Notes Payable [Member] Notes due 2016 at 2.375%	Mar. 31, 2014 Notes Payable [Member] Notes due 2017 at 0.875%	Dec. 31, 2013 Notes Payable [Member] Notes due 2017 at 0.875%	Mar. 31, 2014 Notes Payable [Member] Notes due 2018 at 1.00%	Dec. 31, 2013 Notes Payable [Member] Notes due 2018 at 1.00%	May 31, 2013 Notes Payable [Member] Notes due 2018 at 1.00%	Mar. 31, 2014 Notes Payable [Member] Notes due 2019 at 1.65%	Dec. 31, 2013 Notes Payable [Member] Notes due 2019 at 1.65%	Mar. 31, 2014 Notes Payable [Member] Notes due 2021 at 2.75%	Dec. 31, 2013 Notes Payable [Member] Notes due 2021 at 2.75%	Mar. 31, 2014 Notes Payable [Member] Notes due 2023 at 2.25%	Dec. 31, 2013 Notes Payable [Member] Notes due 2023 at 2.25%	May 31, 2013 Notes Payable [Member] Notes due 2023 at 2.25%	Mar. 31, 2014 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Dec. 31, 2013 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Mar. 31, 2014 National [Member] Notes Payable [Member] Notes due 2017 at 6.60% (assumed with National acquisition)	Dec. 31, 2013 National [Member] Notes Payable [Member] Notes due 2017 at 6.60% (assumed with National acquisition)
Short-term borrowings
Line of credit, borrowing capacity	$ 2,000,000,000
Commercial paper	0
Long-term debt
Long-term debt stated interest rate (percent)						1.375%		0.45%		2.375%		0.875%		1.00%			1.65%		2.75%		2.25%			3.95%		6.60%
Long-term debt, gross	5,625,000,000		5,125,000,000	500,000,000	1,000,000,000	1,000,000,000	1,000,000,000	750,000,000	750,000,000	1,000,000,000	1,000,000,000	250,000,000	0	500,000,000	500,000,000	500,000,000	750,000,000	750,000,000	250,000,000	0	500,000,000	500,000,000	500,000,000	250,000,000	250,000,000	375,000,000	375,000,000
Proceeds from issuance of long-term debt				498,000,000	986,000,000
Repayments of long-term debt					1,500,000,000	1,000,000,000
Payments of debt issuance costs				3,000,000	6,000,000
Net unamortized premium	27,000,000		33,000,000
Current portion of long-term debt	(1,000,000,000)		(1,000,000,000)
Long-term debt	4,652,000,000		4,158,000,000
Interest and debt expense	$ 25,000,000	$ 23,000,000