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LEASES: (Details) - USD ($)
 6 Months Ended
Jun. 30, 2019
Jan. 01, 2019
Lessee, Lease, Description [Line Items]    
The weighted average remaining lease term 10 years  
The weighted average discount rate 3.56%  
Total lease expense $ 57,800,000  
Short term lease costs 300,000  
Maturities of lease liabilities    
2019 57,800,000  
2020 115,400,000  
2021 115,300,000  
2022 112,100,000  
2023 111,100,000  
After 2023 929,100,000  
Total lease payments 1,440,800,000  
Less: interest on lease liabilities (364,500,000)  
Lease liabilities 1,076,300,000 $ 1,115,900,000
Cost of sales (exclusive of depreciation, amortization and depletion)    
Lessee, Lease, Description [Line Items]    
Total lease expense 57,600,000  
Selling, general and administrative    
Lessee, Lease, Description [Line Items]    
Total lease expense 100,000  
Exploration    
Lessee, Lease, Description [Line Items]    
Total lease expense $ 100,000  
Minimum    
Lessee, Lease, Description [Line Items]    
Remaining lease terms 2 years  
Maximum    
Lessee, Lease, Description [Line Items]    
Underlying asset value $ 10,000  
Total nominal contract value $ 100,000  
Remaining lease terms 14 years