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Basis of Presentation - Covid 19 (Details) - USD ($)
$ / shares in Units, $ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Apr. 23, 2020
Debt Instrument [Line Items]        
Cash and cash equivalents - unrestricted $ 71,612 $ 56,697 $ 82,376  
Revolving Credit Facility [Member] | $700 Million Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Remaining borrowing capacity $ 474,700      
Sixth Amended And Restated Credit Agreement [Member]        
Debt Instrument [Line Items]        
Dividends permitted amount per quarter       $ 0.01