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Long-term debt outstanding (Details 3) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
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Long term debt outstanding [Abstract] |  |
Long-term debt | $ 1,790,159 | $ 1,796,163
Long-term debt due within one year | 43,598 | 238,539
Long-term debt | $ 1,746,561 | 1,557,624
Senior notes [Member] |  |
Debt instrument [Line Items] |  |
Weighted average interest rate | 4.87% |
Long term debt outstanding [Abstract] |  |
Long-term debt | $ 1,437,831 | 1,616,246
Commercial paper [Member] |  |
Debt instrument [Line Items] |  |
Weighted average interest rate | 1.27% |
Long term debt outstanding [Abstract] |  |
Long-term debt | $ 262,000 | 62,500
Medium-term notes [Member] |  |
Debt instrument [Line Items] |  |
Weighted average interest rate | 6.68% |
Long term debt outstanding [Abstract] |  |
Long-term debt | $ 50,000 | 50,000
Other notes [Member] |  |
Debt instrument [Line Items] |  |
Weighted average interest rate | 5.25% |
Long term debt outstanding [Abstract] |  |
Long-term debt | $ 24,471 | 24,589
Credit agreements [Member] |  |
Debt instrument [Line Items] |  |
Weighted average interest rate | 3.14% |
Long term debt outstanding [Abstract] |  |
Long-term debt | $ 21,793 | 48,906
Long-term debt [Member] |  |
Long term debt outstanding [Abstract] |  |
Unamortized debt issuance costs | (5,832) | (6,069)
Discount | $ (104) | $ (9)

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