Financial Instruments (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Restricted investment:
Rabbi Trust	$ 19,472	$ 15,662
Fixed income securities	1,917	1,782
Interest rate cap derivatives		15
Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	15,673	18,679
Quoted Prices in Active Markets (Level 1)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest rate cap derivatives		0
Quoted Prices in Active Markets (Level 1) | Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	0	0
Significant Other Observable Inputs (Level 2)
Restricted investment:
Rabbi Trust	19,472	15,662
Fixed income securities	1,917	1,782
Significant Other Observable Inputs (Level 2) | Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	15,673	18,679
Significant Unobservable Inputs (Level 3)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest rate cap derivatives		0
Significant Unobservable Inputs (Level 3) | Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	$ 0	$ 0