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