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Earnings per Share (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Feb. 14, 2020
Nov. 15, 2018
Aug. 30, 2018
Apr. 26, 2016
Feb. 28, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Earnings per Share [Line Items]                
Total anti-dilutive shares excluded from the calculation (in shares)           49,500 42,000 250
Dividend distribution, cash paid           $ 107,722 $ 102,333 $ 61,931
Number of shares repurchased           686,240 0 0
Stock repurchase program, period       24 months        
Number of shares repurchased, cost           $ 25,173    
Stock repurchase program, remaining amount           $ 74,800    
Unsettled repurchase shares           0 0  
Maximum [Member]                
Earnings per Share [Line Items]                
Stock repurchase program, authorized amount     $ 100,000 $ 60,000        
Deltic [Member]                
Earnings per Share [Line Items]                
Number of shares issued upon merger         22,000,000.0      
Deltic [Member] | Subsequent Event [Member]                
Earnings per Share [Line Items]                
Dividend payable, per share $ 0.40              
Distribution, date of declared Feb. 14, 2020              
Distribution, date of record Mar. 31, 2020              
Deltic [Member] | Special Distribution [Member]                
Earnings per Share [Line Items]                
Number of shares issued upon merger   4,800,000            
Distribution payable     $ 222,000          
Dividend payable, per share     $ 3.54          
Distribution, date of declared     Aug. 30, 2018          
Distribution, date to be paid     Nov. 15, 2018          
Distribution, date of record     Sep. 27, 2018          
Dividend distribution, cash paid   $ 44,400