Debt and lines of credit (Details) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Mar. 31, 2014 Notes Payable	May 31, 2013 Notes Payable	May 31, 2014 Notes Payable Notes due 2014 at 1.375%	Sep. 30, 2014 Notes Payable Notes due 2014 at 1.375%	Dec. 31, 2013 Notes Payable Notes due 2014 at 1.375%	Sep. 30, 2014 Notes Payable Notes due 2015 at 0.45%	Dec. 31, 2013 Notes Payable Notes due 2015 at 0.45%	Sep. 30, 2014 Notes Payable Notes due 2016 at 2.375%	Dec. 31, 2013 Notes Payable Notes due 2016 at 2.375%	Sep. 30, 2014 Notes Payable Notes due 2017 at 0.875%	Mar. 31, 2014 Notes Payable Notes due 2017 at 0.875%	Dec. 31, 2013 Notes Payable Notes due 2017 at 0.875%	Sep. 30, 2014 Notes Payable Notes due 2018 at 1.00%	Dec. 31, 2013 Notes Payable Notes due 2018 at 1.00%	May 31, 2013 Notes Payable Notes due 2018 at 1.00%	Sep. 30, 2014 Notes Payable Notes due 2019 at 1.65%	Dec. 31, 2013 Notes Payable Notes due 2019 at 1.65%	Sep. 30, 2014 Notes Payable Notes due 2021 at 2.75%	Mar. 31, 2014 Notes Payable Notes due 2021 at 2.75%	Dec. 31, 2013 Notes Payable Notes due 2021 at 2.75%	Sep. 30, 2014 Notes Payable Notes due 2023 at 2.25%	Dec. 31, 2013 Notes Payable Notes due 2023 at 2.25%	May 31, 2013 Notes Payable Notes due 2023 at 2.25%	Sep. 30, 2014 National Notes Payable Notes due 2015 at 3.95% (assumed with National acquisition)	Dec. 31, 2013 National Notes Payable Notes due 2015 at 3.95% (assumed with National acquisition)	Sep. 30, 2014 National Notes Payable Notes due 2017 at 6.60% (assumed with National acquisition)	Dec. 31, 2013 National Notes Payable Notes due 2017 at 6.60% (assumed with National acquisition)
Short-term borrowings
Line of credit, borrowing capacity	$ 2,000,000,000		$ 2,000,000,000
Commercial paper	0		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	4,625,000,000		4,625,000,000		5,125,000,000	500,000,000	1,000,000,000		0	1,000,000,000	750,000,000	750,000,000	1,000,000,000	1,000,000,000	250,000,000	250,000,000	0	500,000,000	500,000,000	500,000,000	750,000,000	750,000,000	250,000,000	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		498,000,000	986,000,000
Repayments of long-term debt			1,000,000,000	1,500,000,000			1,500,000,000	1,000,000,000
Payments of debt issuance costs						3,000,000	6,000,000
Net unamortized premium	20,000,000		20,000,000		33,000,000
Current portion of long-term debt	(1,002,000,000)		(1,002,000,000)		(1,000,000,000)
Long-term debt	3,643,000,000		3,643,000,000		4,158,000,000
Interest and debt expense	$ 23,000,000	$ 24,000,000	$ 72,000,000	$ 71,000,000