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Earnings per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended
Aug. 30, 2018
Feb. 28, 2018
Mar. 31, 2019
Mar. 31, 2018
Earnings per Share [Line Items]        
Total anti-dilutive shares excluded from the calculation (in shares)     90,000 64,000
Dividend distribution, cash     $ 27,065 $ 25,102
Number of shares repurchased     278,947  
Number of shares repurchased, cost     $ 10,158  
Stock repurchase program, remaining amount     $ 89,900  
Unsettled repurchase shares     0  
Maximum [Member]        
Earnings per Share [Line Items]        
Stock repurchase program, authorized amount $ 100,000      
Deltic [Member]        
Earnings per Share [Line Items]        
Number of shares issued upon merger   22,000,000    
Deltic [Member] | Special Distribution [Member]        
Earnings per Share [Line Items]        
Number of shares issued upon merger     4,814,673  
Distribution payable $ 222,000      
Dividend payable, per share $ 3.54      
Dividend distribution, cash     $ 44,400  
Distribution, date of declared Aug. 30, 2018      
Distribution, date to be paid Nov. 15, 2018      
Distribution, date of record Sep. 27, 2018