Financial Instruments (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Restricted investment:
Rabbi Trust	$ 12,459	$ 11,281
Fixed income securities	1,778	1,966
Interest rate cap derivatives assets	210	570
Quoted Prices in Active Markets (Level 1)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest rate cap derivatives assets	0
Significant Other Observable Inputs (Level 2)
Restricted investment:
Rabbi Trust	12,459	11,281
Fixed income securities	1,778	1,966
Interest rate cap derivatives assets	210	570
Significant Unobservable Inputs (Level 3)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest rate cap derivatives assets	0
Interest rate swap derivatives
Liabilities:
Interest rate swap derivatives	22,113	19,248
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	22,113	19,248
Interest rate swap derivatives | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivatives	$ 0	$ 0