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Earnings Per Common Share - Textual (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 12 Months Ended
Feb. 23, 2018
Jan. 16, 2018
Oct. 20, 2017
Oct. 16, 2017
Jul. 20, 2017
Oct. 17, 2016
Jul. 21, 2016
Oct. 15, 2015
Jul. 16, 2015
Apr. 22, 2015
Oct. 15, 2014
Jul. 17, 2014
Sep. 30, 2009
Jan. 17, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Apr. 23, 2015
Feb. 14, 2014
Dec. 31, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                        
Antidilutive shares excluded from computation of diluted weighted-average shares (shares)                             200,000 100,000        
Common stock repurchased                             $ 38.1 $ 65.6 $ 45.3      
Cash dividends declared per common share (in dollars per share)     $ 0.25   $ 0.25   $ 0.24   $ 0.23     $ 0.21     $ 0.98 $ 0.94 $ 0.88      
Cash dividends paid per common share (dollars per share)       $ 0.25   $ 0.24   $ 0.23     $ 0.21                  
Preferred stock conversion rate (shares for each share)                                   $ 33.9905    
Common stock, par value (in dollars per share)                             $ 0.01 $ 0.01   $ 0.01    
Common stock issued (shares)                         5,000,000 700,000            
Common stock warrants issued                         8,200,000 0   8,200,000        
Exercise price of warrants (in dollars per share)                         $ 41.40 $ 41.40   $ 41.40        
Proceeds from exercising stock warrants                           $ 26.7 $ 26.6 $ 0.0 $ 0.0      
Subsequent Event [Member]                                        
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                        
Cash dividends declared per common share (in dollars per share) $ 0.25                                      
Cash dividends paid per common share (dollars per share)   $ 0.25                                    
Common Stock [Member]                                        
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                        
Common stock repurchase authorization                                     $ 250.0 $ 200.0
Common stock repurchased (shares)                             900,000 1,700,000 1,300,000      
Common stock repurchased                             $ 38.1 $ 65.6 $ 45.3      
Accrued common stock dividends                             $ 25.4 $ 22.2        
Common stock issued and delivered (shares)                   3,271,415                    
Convertible Preferred Stock [Member]                                        
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                                        
Convertible preferred stock par value (in dollars per share)                   $ 0.10                    
Preferred stock liquidation preference (in dollars per share)                   $ 1,000                    
Preferred stock outstanding (shares)                                   96,245