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Debt (Narrative) (Details) - USD ($)
1 Months Ended
Dec. 31, 2018
Jan. 31, 2019
Mar. 31, 2019
Debt Instrument [Line Items]      
Interest Rate   4.56%  
Long-term Debt, refinancing amount   $ 150,000,000  
Debt instrument maturity date   2029  
Debt Instrument, outstanding amount including redemption premium   $ 4,900,000  
Debt Instrument, lender fee   500,000  
Revenue Bonds [Member] | Union County, Arkansas [Member] | Deltic [Member]      
Debt Instrument [Line Items]      
Debt instrument maturity date Oct. 01, 2027    
Revenue Bonds [Member] | Union County, Arkansas [Member] | Letter of Credit [Member] | Deltic [Member]      
Debt Instrument [Line Items]      
Debt assumed $ 29,000,000    
Letter of Credit [Member]      
Debt Instrument [Line Items]      
Line of credit facility, amount outstanding     $ 1,000,000
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Maximum Borrowing Capacity     $ 380,000,000
Amended Term Loan Agreement [Member]      
Debt Instrument [Line Items]      
Line of credit facility, amount outstanding   $ 693,500,000  
London Interbank Offered Rate (LIBOR) [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate   1.85%  
Senior Notes [Member]      
Debt Instrument [Line Items]      
Proceeds from Issuance of Debt   $ 150,000,000  
Interest Rate   7.50%