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Cumulative Preferred Stock (Characteristics of the First Preferred Shares) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2013
Series A Preferred Stock [Member]    
Class of Stock [Line Items]    
Initial Yield 4.40%  
Annual Dividend Per Share $ 0.6388  
Earliest Redemption and/or Conversion Option Date Aug. 15, 2020  
Preferred Stock, Redemption Price Per Share $ 25  
Right to Convert on a one for one basis 1  
Series A Preferred Stock [Member] | Minimum [Member]    
Class of Stock [Line Items]    
Initial Yield 1.84%  
Series C Preferred Stock [Member]    
Class of Stock [Line Items]    
Initial Yield 4.10%  
Annual Dividend Per Share $ 1.1802 $ 1.025
Earliest Redemption and/or Conversion Option Date Aug. 15, 2023  
Preferred Stock, Redemption Price Per Share $ 25  
Right to Convert on a one for one basis 1  
Series C Preferred Stock [Member] | Minimum [Member]    
Class of Stock [Line Items]    
Initial Yield 2.65%  
Series F Preferred Stock [Member]    
Class of Stock [Line Items]    
Initial Yield 4.25%  
Annual Dividend Per Share $ 1.0625  
Earliest Redemption and/or Conversion Option Date Feb. 15, 2020  
Preferred Stock, Redemption Price Per Share $ 25  
Right to Convert on a one for one basis 1  
Series F Preferred Stock [Member] | Minimum [Member]    
Class of Stock [Line Items]    
Initial Yield 2.63%  
Series B Preferred Stock [Member]    
Class of Stock [Line Items]    
Initial Yield 2.393%  
Earliest Redemption and/or Conversion Option Date Aug. 15, 2020  
Preferred Stock, Redemption Price Per Share $ 25  
Right to Convert on a one for one basis 1  
Series B Preferred Stock [Member] | Minimum [Member]    
Class of Stock [Line Items]    
Initial Yield 1.84%  
Series H Preferred Stock [Member}    
Class of Stock [Line Items]    
Initial Yield 4.90%  
Annual Dividend Per Share $ 1.225  
Earliest Redemption and/or Conversion Option Date Aug. 15, 2023  
Preferred Stock, Redemption Price Per Share $ 25  
Right to Convert on a one for one basis 1  
Series H Preferred Stock [Member} | Minimum [Member]    
Class of Stock [Line Items]    
Initial Yield 4.90%  
Series I Preferred Stock [Member] | Minimum [Member]    
Class of Stock [Line Items]    
Initial Yield 2.54%  
Perpetual fixed rate Series E Preferred Stock [Member]    
Class of Stock [Line Items]    
Initial Yield 4.50%  
Annual Dividend Per Share $ 1.125  
Preferred Stock, Redemption Price Per Share $ 26