Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Liabilities:
Accounts receivable, transferred to third party	$ 3,000	$ 6,900
Proceeds from sale and collection of receivables	3,000	6,900
Recurring
Restricted investments:
Rabbi Trust	28,332	20,892
Fixed income securities	1,892	1,801
Recurring | Quoted Prices in Active Markets (Level 1)
Restricted investments:
Rabbi Trust	0	0
Fixed income securities	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Restricted investments:
Rabbi Trust	28,332	20,892
Fixed income securities	1,892	1,801
Recurring | Significant Unobservable Inputs (Level 3)
Restricted investments:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest Rate Swap | Recurring
Liabilities:
Interest rate swap derivative liability	1,869	8,638
Interest Rate Swap | Recurring | Quoted Prices in Active Markets (Level 1)
Liabilities:
Interest rate swap derivative liability	0	0
Interest Rate Swap | Recurring | Significant Other Observable Inputs (Level 2)
Liabilities:
Interest rate swap derivative liability	1,869	8,638
Interest Rate Swap | Recurring | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivative liability	$ 0	$ 0