Financial Instruments (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Restricted investments:
Rabbi Trust	$ 15,662	$ 13,071
Fixed income securities	1,782	1,717
Interest rate swap derivative assets	15	93
Quoted Prices in Active Markets (Level 1)
Restricted investments:
Rabbi Trust		0
Fixed income securities		0
Interest rate swap derivative assets	0	0
Significant Other Observable Inputs (Level 2)
Restricted investments:
Rabbi Trust	15,662	13,071
Fixed income securities	1,782	1,717
Interest rate swap derivative assets	15	93
Significant Unobservable Inputs (Level 3)
Restricted investments:
Rabbi Trust		0
Fixed income securities		0
Interest rate swap derivative assets	0
Interest Rate Swap
Liabilities:
Interest rate swap derivative liability	18,679	20,835
Interest Rate Swap | Quoted Prices in Active Markets (Level 1)
Liabilities:
Interest rate swap derivative liability		0
Interest Rate Swap | Significant Other Observable Inputs (Level 2)
Liabilities:
Interest rate swap derivative liability	$ 18,679	20,835
Interest Rate Swap | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivative liability		$ 0