Fair Value Measurements of Assets and Liabilities on Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted funds	$ 37,283	$ 29,259
Rabbi trust investments	666	444
Deposits	1,318	1,901
Mark-to-market derivative asset	3,515	4,776
Total assets	42,782	36,380
Deferred compensation obligation	11,548	11,928
Mark-to-market derivative liability	1,068	1,276
Total liabilities	12,616	13,204
Total net assets (liabilities)	30,166	23,176
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted funds	37,283	29,259
Rabbi trust investments	0	0
Deposits	1,318	1,901
Mark-to-market derivative asset	0	0
Total assets	38,601	31,160
Deferred compensation obligation	0	0
Mark-to-market derivative liability	0	0
Total liabilities	0	0
Total net assets (liabilities)	38,601	31,160
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted funds	0	0
Rabbi trust investments	666	444
Deposits	0	0
Mark-to-market derivative asset	3,515	4,776
Total assets	4,181	5,220
Deferred compensation obligation	11,548	11,928
Mark-to-market derivative liability	1,068	1,276
Total liabilities	12,616	13,204
Total net assets (liabilities)	(8,435)	(7,984)
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted funds	0	0
Rabbi trust investments	0	0
Deposits	0	0
Mark-to-market derivative asset	0	0
Total assets	0	0
Deferred compensation obligation	0	0
Mark-to-market derivative liability	0	0
Total liabilities	0	0
Total net assets (liabilities)	$ 0	$ 0