Financial Instruments (Schedule of Long-Term Debt) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 520,477	$ 515,919
Less: current portion	(1,821)	(1,799)
Long-term debt, net of current portion	518,656	514,120
Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term debt	$ 506,000	$ 501,000
Interest rate	3.49%	3.40%
Maturity date range, end	Dec. 31, 2022
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Long-term debt	$ 14,477	$ 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.

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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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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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