Financial Instruments (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Restricted investment:
Rabbi Trust	$ 14,664	$ 13,071
Fixed income securities	1,845	1,717
Interest rate cap derivatives assets	47	93
Quoted Prices in Active Markets (Level 1)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest rate cap derivatives assets	0	0
Significant Other Observable Inputs (Level 2)
Restricted investment:
Rabbi Trust	14,664	13,071
Fixed income securities	1,845	1,717
Interest rate cap derivatives assets	47	93
Significant Unobservable Inputs (Level 3)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest rate cap derivatives assets	0	0
Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	23,706	20,835
Interest rate swap derivatives | Quoted Prices in Active Markets (Level 1)
Liabilities:
Interest rate swap derivatives	0	0
Interest rate swap derivatives | Significant Other Observable Inputs (Level 2)
Liabilities:
Interest rate swap derivatives	23,706	20,835
Interest rate swap derivatives | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivatives	$ 0	$ 0