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Quarterly Data (Unaudited) (Details) - $ / shares
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Condensed consolidated quarterly financial information [Abstract]                      
Basic EPS - Net income (in dollars per share) $ 0 $ 0.63 $ 0.65 $ 0.21 $ 0.50 $ 0.46 $ 0.64 $ 0.36 $ 1.49 $ 1.95 $ 3.51
Diluted EPS - Net income (in dollars per share) 0 0.62 0.65 0.21 0.50 0.45 0.64 0.35 1.48 1.94 3.47
Dividends per share of common stock (in dollars per share) 0.35 0.35 0.35 0.35 0.3125 0.3125 0.3125 0.3125 $ 1.40 $ 1.25 $ 1.11
High common stock sales price (in dollars per share) 83.34 74.82 65.56 70.84 61.25 58.36 52.23 48.89      
Low common stock sales price (in dollars per share) $ 69.63 $ 59.58 $ 53.26 $ 43.70 $ 55.17 $ 50.27 $ 46.82 $ 42.17