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Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statement of Stockholders' Equity      
Dividends declared (in dollars per share) $ 1.26 $ 1.24 $ 1.24
Common stock grants awarded, shares 2,362    
Share retired, shares 32 16,463 137,752