Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivative Instrument (Assets Measured at Fair Value on a Recurring Basis) (Details)

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Fair Value Measurements and Derivative Instrument (Assets Measured at Fair Value on a Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation plan assets (mutual funds) $ 1,323 $ 1,477
Derivative stock appreciation rights 78,330 29,700
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation plan assets (mutual funds) 1,323 1,477
Derivative stock appreciation rights 0 0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation plan assets (mutual funds) 0 0
Derivative stock appreciation rights 0 0
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation plan assets (mutual funds) 0 0
Derivative stock appreciation rights $ 78,330 $ 29,700