Financial Instruments (Schedule of Long-Term Debt) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 530,234	$ 515,919
Less: current portion	(1,231)	(1,799)
Long-term debt, net of current portion	529,003	514,120
Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term debt	$ 504,000	$ 501,000
Interest rate	3.61%	3.40%
Maturity date range, end	Dec. 31, 2022
Finance lease obligation [Member]
Debt Instrument [Line Items]
Long-term debt	$ 26,234	$ 14,919
Interest rate	5.00%
Maturity date range, end	Dec. 31, 2022

Contractual interest rate for funds borrowed, under the debt agreement.

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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