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Consolidated Statements of Changes in Common Shareholders' Equity (Parentheticals) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Consolidated Statements Of Changes In Common Shareholders' Equity [Abstract]      
Cash dividends declared per share $ 3.88 $ 3.70 $ 3.06