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Indebtedness Unsecured Debt (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Long-term Debt $ 3,080,262 $ 2,908,477
Variable Rate UnSecured Debt [Member] [Member]    
Debt Instrument [Line Items]    
Long-term Debt $ 250,000  
Debt Instrument, Interest Rate, Effective Percentage 1.99%  
Fixed Rate Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Estimated Trading Values, Percent of Face Value, Minimum 96.00%  
Debt Instrument, Estimated Trading Values, Percent of Face Value, Maximum 127.00%  
Debt Instrument, Covenant Compliance We were in compliance with all such financial covenants at March 31, 2017  
Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Long-term Debt $ 2,498,220 $ 2,498,835