Long-term debt outstanding (Details 4) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Senior notes [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	5.74%
Commercial paper [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	0.51%
Medium-term notes [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	7.58%
Other notes [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	5.24%
Loans payable [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	5.22%
Long-term debt [Member]
Long term debt outstanding [Abstract]
Long-term debt	$ 1,744,975,000	$ 1,424,678,000
Less current maturities	134,108,000	139,267,000
Long-term debt	1,610,867,000	1,285,411,000
Long-term debt maturities [Abstract]
2013	134,100,000
2014	9,300,000
2015	266,500,000
2016	288,500,000
2017	336,400,000
Thereafter	710,200,000
Long-term debt [Member] | Senior notes [Member]
Long term debt outstanding [Abstract]
Long-term debt	1,349,160,000	1,287,576,000
Long-term debt [Member] | Commercial paper [Member]
Long term debt outstanding [Abstract]
Long-term debt	293,000,000	0
Long-term debt [Member] | Medium-term notes [Member]
Long term debt outstanding [Abstract]
Long-term debt	59,000,000	81,000,000
Long-term debt [Member] | Other notes [Member]
Long term debt outstanding [Abstract]
Long-term debt	40,090,000	40,469,000
Long-term debt [Member] | Loans payable [Member]
Long term debt outstanding [Abstract]
Long-term debt	3,768,000	15,633,000
Long-term debt [Member] | Discount on commercial paper program [Member]
Long term debt outstanding [Abstract]
Long-term debt	$ (43,000)	$ 0