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Earnings per Common Share - Textual (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Oct. 26, 2018
Oct. 15, 2018
Jul. 24, 2018
Jul. 16, 2018
May 04, 2018
Apr. 16, 2018
Feb. 23, 2018
Jan. 16, 2018
Oct. 20, 2017
Sep. 30, 2009
Jan. 17, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                  
Cash dividends per common share, Declared (in dollars per share)                 $ 0.25     $ 0.27 $ 0.25 $ 0.77 $ 0.73    
Stock warrants exercised (shares)                   5.0 0.7            
Class of Warrant or Right, Outstanding                   8.2 0.0           8.2
Exercise price of warrants (in dollars per share)                   $ 41.40 $ 41.40           $ 41.40
Proceeds from exercising stock warrants                     $ 27.0     $ 0.0 $ 26.6    
Common Stock                                  
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                  
Cash dividends per common share, Declared (in dollars per share)     $ 0.27   $ 0.25   $ 0.25                    
Cash dividends per common share, Paid (in dollars per share)       $ 0.25   $ 0.25   $ 0.25                  
Common stock dividends accrued                       $ 28.2   $ 28.2   $ 25.4  
Subsequent Event [Member] | Common Stock                                  
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                  
Cash dividends per common share, Declared (in dollars per share) $ 0.27                                
Cash dividends per common share, Paid (in dollars per share)   $ 0.27