Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Liabilities:
Accounts receivable, transferred to third party	$ 3,000
Proceeds from sale of receivables	3,000
Recurring
Restricted investment:
Rabbi Trust	21,268	$ 20,763
Fixed income securities	1,867	1,902
Recurring | Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	13,126	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	21,268	20,763
Fixed income securities	1,867	1,902
Recurring | Significant Other Observable Inputs (Level 2) | Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	13,126	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