Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)

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Fair Value Measurements Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable security gains (losses), net $ 3,100 $ (3,800)
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 144,144  
Marketable securities 33,384  
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  
Marketable securities 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  
Marketable securities 0  
Construction reserve funds $ 0