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Long-term Debt and Interest Expense (Narrative) (Details)
12 Months Ended
Dec. 31, 2019
USD ($)
Rate
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]      
Line of credit facility, maximum borrowing capacity | $ $ 1,500,000,000    
Outstanding borrowings under the Credit Facility | $ 0    
Repayments of debt | $ $ 0 $ 450,000,000 $ 485,000,000
Credit Facility      
Debt Instrument [Line Items]      
Federal fund rate 0.50%    
Alternate base rate spread 0.25%    
Applicable margin 1.25%    
Letters of credit outstanding under the Credit Facility, interest percentage 0.125%    
Unused portion, fee percentage 0.15%    
Debt instrument covenant description 0.65    
Swing Line Loans | Credit Facility      
Debt Instrument [Line Items]      
Maximum outstanding borrowings under the Credit Facility | $ $ 150,000,000