Financial Instruments (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Restricted investments:
Rabbi Trust	$ 20,763	$ 15,662
Fixed income securities	1,902	1,782
Quoted Prices in Active Markets (Level 1)
Restricted investments:
Rabbi Trust	0	0
Fixed income securities	0	0
Significant Other Observable Inputs (Level 2)
Restricted investments:
Rabbi Trust	20,763	15,662
Fixed income securities	1,902	1,782
Significant Unobservable Inputs (Level 3)
Restricted investments:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest Rate Swap
Liabilities:
Interest rate swap derivative liability	13,992	18,679
Interest Rate Swap | Quoted Prices in Active Markets (Level 1)
Liabilities:
Interest rate swap derivative liability	0	0
Interest Rate Swap | Significant Other Observable Inputs (Level 2)
Liabilities:
Interest rate swap derivative liability	13,992	18,679
Interest Rate Swap | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivative liability	$ 0	$ 0