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Notes Payable, Current Maturities and Debt: Schedule of Short-term Debt and Narrative (Details)
6 Months Ended
Jun. 30, 2019
USD ($)
credit_extension
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Short-term Debt [Line Items]      
Notes payable $ 102,500,000   $ 185,620,000
Letters of Credit 10,429,000   22,311,000
Commercial paper, maximum borrowing capacity 750,000,000    
Net (payments) borrowings of short-term debt (83,120,000) $ (89,500,000)  
Commercial Paper      
Short-term Debt [Line Items]      
Notes payable $ 102,500,000   185,620,000
Debt instrument, term 397 days    
Net (payments) borrowings of short-term debt $ (83,000,000)    
Short-term interest rate 2.60%    
Revolving Credit Facility      
Short-term Debt [Line Items]      
Notes payable $ 0   0
Letters of Credit 10,429,000   $ 22,311,000
Current borrowing capacity $ 750,000,000    
Number of one-year extension options | credit_extension 2    
Debt instrument, term 1 year    
Maximum borrowing capacity $ 1,000,000,000.0    
Commitment fee 0.175%    
Revolving Credit Facility | Base Rate      
Short-term Debt [Line Items]      
Interest rate 0.125%    
Revolving Credit Facility | Eurodollar      
Short-term Debt [Line Items]      
Interest rate 1.125%    
Revolving Credit Facility | Letter of Credit      
Short-term Debt [Line Items]      
Interest rate 1.125%