Quarterly report pursuant to Section 13 or 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
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Construction Reserve Funds $ 36,790   $ 51,339
Marketable security gains (losses), net (3,800) $ 12,100  
Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 38,963    
Construction Reserve Funds 36,790    
Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 0    
Construction Reserve Funds 0    
Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 0    
Construction Reserve Funds $ 0