Long-term debt outstanding (Details 4) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Long-term debt maturities [Abstract]
2016	$ 243,800
2017	51,000
2018	175,200
2019	119,700
2020	21,000
Thereafter	$ 1,260,500
Senior notes [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	5.18%
Commercial paper [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	0.73%
Term loan agreements [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	2.16%
Medium-term notes [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	6.68%
Other notes [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	5.25%
Loans payable [Member]
Long term debt outstanding [Abstract]
Weighted average interest rate	1.82%
Long-term debt [Member]
Long term debt outstanding [Abstract]
Long-term debt	$ 1,871,232	$ 2,093,830
Less current maturities	243,789	268,552
Long-term debt	1,627,443	1,825,278
Long-term debt [Member] \| Senior notes [Member]
Long term debt outstanding [Abstract]
Long-term debt	1,616,246	1,636,662
Long-term debt [Member] \| Commercial paper [Member]
Long term debt outstanding [Abstract]
Long-term debt	62,500	288,500
Long-term debt [Member] \| Term loan agreements [Member]
Long term debt outstanding [Abstract]
Long-term debt	69,000	72,000
Long-term debt [Member] \| Medium-term notes [Member]
Long term debt outstanding [Abstract]
Long-term debt	50,000	35,000
Long-term debt [Member] \| Other notes [Member]
Long term debt outstanding [Abstract]
Long-term debt	24,589	39,662
Long-term debt [Member] \| Loans payable [Member]
Long term debt outstanding [Abstract]
Long-term debt	48,906	22,042
Long-term debt [Member] \| Discount on commercial paper program [Member]
Long term debt outstanding [Abstract]
Long-term debt	$ (9)	$ (36)

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Topic 210
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Paragraph 20, 22
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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