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Indebtedness Unsecured Debt (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2019
Dec. 31, 2017
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 3.38%  
Issuance Price of Notes as a Percentage of Par Value 104.16%  
Debt Instrument, Interest Rate, Effective Percentage 2.80%  
Document Period End Date Sep. 30, 2019  
Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Estimated Trading Values, Percent of Face Value, Minimum 102.00%  
Debt Instrument, Estimated Trading Values, Percent of Face Value, Maximum 131.00%  
Debt Instrument, Covenant Compliance We were in compliance with all such financial covenants at  
Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount $ 175.0 $ 300.0