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Unsecured Debt - Future Obligations (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Total Debt $ 2,008,417 $ 2,191,481
Unsecured Debt [Member]    
Debt Instrument [Line Items]    
2018 0  
2019 75,000  
2020 0  
2021 500,000  
2022 650,000  
Thereafter 2,300,000  
Total Debt $ 3,525,000