Financial Instruments (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Restricted investments:
Rabbi Trust	$ 13,071	$ 11,281
Fixed income securities	1,717	1,966
Interest rate swap derivative assets	93	570
Quoted Prices in Active Markets (Level 1)
Restricted investments:
Rabbi Trust		0
Fixed income securities		0
Interest rate swap derivative assets	0
Significant Other Observable Inputs (Level 2)
Restricted investments:
Rabbi Trust	13,071	11,281
Fixed income securities	1,717	1,966
Interest rate swap derivative assets	93	570
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	20,835	19,248
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	20,835	19,248
Interest Rate Swap | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivative liability	$ 0	$ 0