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Fair Value of Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Liabilities:    
Senior unsecured notes $ 1,724,936 $ 1,722,758
Carrying Amount    
Liabilities:    
Senior unsecured notes 1,724,936 1,722,758
Secured debt 804,608 1,106,183
Debt instrument 2,529,544 2,828,941
Estimated Fair Value    
Liabilities:    
Senior unsecured notes 1,823,535 1,755,715
Secured debt 782,175 1,090,515
Debt instrument $ 2,605,710 $ 2,846,230