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, 2019
Dec. 31, 2018
Dec. 31, 2017
Marketable Security Positions Held By The Company As Of December 31, 2019      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable security gains (losses), net $ 2,200    
Marketable Security Positions Held By The Company As Of December 31, 2018      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable security gains (losses), net   $ (12,400) $ 100
Fair Value, Measurements, Recurring | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash, cash equivalents, restricted cash and restricted cash equivalents 78,444 147,212  
Marketable securities 7,936 30,316  
Construction reserve funds   3,908  
Fair Value, Measurements, Recurring | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash, cash equivalents, restricted cash and restricted cash equivalents 0 0  
Marketable securities 0 0  
Construction reserve funds   0  
Fair Value, Measurements, Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash, cash equivalents, restricted cash and restricted cash equivalents 0 0  
Marketable securities $ 0 0  
Construction reserve funds   $ 0