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Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Common dividends, per share $ 1.60 $ 1.525 $ 1.50
Deltic [Member] | Special Distribution [Member]      
Common dividends, per share $ 3.54