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Financial Instruments - Credit Risk Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Concentration Risk [Line Items]      
Maximum exposure to credit risk $ 1,403,491,000 $ 1,586,978,000  
Accounts receivable [1] $ 8,928,000 4,638,000  
Accounts receivable, threshold period past due, writeoff 120 days    
Current expected credit loss $ 74,000 136,000  
Cash and cash equivalents 1,078,023,000 1,199,693,000  
Short-term investments 211,766,000 306,347,000  
Advances to joint ventures 467,000 19,437,000 $ 4,689,000
Advances to joint ventures, allowance for credit loss $ 0 $ 0  
Four Customers | Accounts Receivable | Credit Concentration Risk      
Concentration Risk [Line Items]      
Concentration risk, percentage 78.00%    
Two Customers | Accounts Receivable | Credit Concentration Risk      
Concentration Risk [Line Items]      
Concentration risk, percentage   56.00%  
[1] Net of current expected credit loss allowance of $74 as of December 31, 2020 (December 31, 2019 – $136)