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Basis of Presentation - Covid 19 (Details) - USD ($)
$ / shares in Units, $ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Apr. 23, 2020
Mar. 31, 2020
Debt Instrument [Line Items]        
Cash and cash equivalents - unrestricted $ 67,138 $ 56,697   $ 662,156
Revolving Credit Facility [Member] | $700 Million Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Remaining borrowing capacity $ 699,700      
Sixth Amended And Restated Credit Agreement [Member]        
Debt Instrument [Line Items]        
Dividends permitted amount per quarter     $ 0.01