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Financial Instruments - Credit Risk Narrative (Details) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Concentration Risk [Line Items]    
Maximum exposure to credit risk $ 1,424,786,000 $ 1,586,978,000
Accounts receivable, threshold period past due, writeoff 120 days  
Current expected credit loss $ 80,000 136,000
Cash and cash equivalents 1,109,700,000 1,199,693,000
Short-term investments 213,614,000 306,347,000
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%