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
12 Months Ended
Dec. 31, 2016
Jun. 30, 2017
Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable Securities, Unrealized Gain (Loss) $ (100)    
Construction Reserve Funds 78,209 $ 67,799  
Conversion Option Liability on 3.75% Convertible Senior Notes $ 0 27,109  
Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities     $ 688
Derivative instruments (included in other receivables)     0
Construction Reserve Funds     67,799
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
Derivative instruments (included in other receivables)     225
Construction Reserve Funds     0
Derivative instruments (included in other current liabilities)     623
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
Derivative instruments (included in other receivables)     0
Construction Reserve Funds     0
Derivative instruments (included in other current liabilities)     $ 0
Three Point Seven Five Percentage Convertible Notes [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Embedded Derivative, Fair Value of Embedded Derivative Liability   $ 27,300  
Debt Instrument, Interest Rate, Stated Percentage   3.75%