Financial Instruments (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Accounts receivable, transferred to third party	$ 6,900
Proceeds from sale of receivables	6,900
Recurring
Restricted investment:
Rabbi Trust	23,176	$ 20,763
Fixed income securities	1,871	1,902
Recurring \| Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	8,638	13,992
Recurring \| Quoted Prices in Active Markets (Level 1)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Recurring \| Quoted Prices in Active Markets (Level 1) \| Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	0	0
Recurring \| Significant Other Observable Inputs (Level 2)
Restricted investment:
Rabbi Trust	23,176	20,763
Fixed income securities	1,871	1,902
Recurring \| Significant Other Observable Inputs (Level 2) \| Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	8,638	13,992
Recurring \| Significant Unobservable Inputs (Level 3)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Recurring \| Significant Unobservable Inputs (Level 3) \| Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	$ 0	$ 0

Accounts Receivable, Transferred To Third Party

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- Definition

Rabbi trust.

+ References

No definition available.

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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- References

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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- Definition

The cash inflow associated with the proceeds from sale and collection of receivables during the period.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
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- References

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