Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total long-term Debt	$ 1,615,405	$ 1,837,372
Current portion of long-term debt	208,681	422,012
Long-term debt	1,406,724	1,415,360
Senior Notes | 1.875% Senior Notes
Debt Instrument [Line Items]
Total long-term Debt	452,958	463,575
Senior Notes | 4.15% Senior Notes
Debt Instrument [Line Items]
Total long-term Debt	421,872	421,628
Senior Notes | 4.50% Senior Notes
Debt Instrument [Line Items]
Total long-term Debt	174,475	174,325
Senior Notes | 5.45% Senior Notes
Debt Instrument [Line Items]
Total long-term Debt	345,918	345,841
Commercial paper notes
Debt Instrument [Line Items]
Total long-term Debt	208,000	421,321
Variable-rate foreign bank loans
Debt Instrument [Line Items]
Total long-term Debt	7,256	5,298
Other long-term debt
Debt Instrument [Line Items]
Total long-term Debt	$ 4,926	$ 5,384