Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Financial Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details)

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Fair Value Measurements (Financial Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Construction Reserve Funds $ 51,339 $ 75,753  
Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 42,761 76,137  
Construction Reserve Funds 51,339 75,753  
Short sales of marketable securities   1,274  
Derivative instruments (included in other current liabilities)   0  
Embedded Derivative, Fair Value of Embedded Derivative Liability   0  
Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 0 0  
Construction Reserve Funds 0 0  
Short sales of marketable securities   0  
Derivative instruments (included in other current liabilities)   36  
Embedded Derivative, Fair Value of Embedded Derivative Liability   0  
Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 0 0  
Construction Reserve Funds 0 0  
Short sales of marketable securities   0  
Derivative instruments (included in other current liabilities)   0  
Embedded Derivative, Fair Value of Embedded Derivative Liability   19,436  
Marketable Security Positions Held By The Company As Of December 31, 2017 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable security gains (losses), net include losses $ 100 (18,500) $ 3,100
Marketable Security Positions Held By The Company As Of December 31, 2016 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable security gains (losses), net include losses   $ (32,200) $ 5,300