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Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retained Earnings      
Cash dividends declared on common shares, per share $ 1.1247 $ 0.6808 $ 0.6862
Serial Preferred Stock      
Preference Shares authorized 20,000,000.0 20,000,000.0 20,000,000.0
Preference Shares issued 0 0 0
Preferred Shares outstanding 0 0 0
Voting Preferred Stock      
Preference Shares authorized 5,000,000 5,000,000 5,000,000
Preference Shares issued 0 0 0