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Fair Value of Assets and Liabilities - Liabilities (Details)
$ in Thousands
Mar. 31, 2018
USD ($)
debt_security
Dec. 31, 2017
USD ($)
Liabilities:    
Senior unsecured notes $ 2,213,811 $ 1,725,662
Carrying Amount    
Liabilities:    
Senior unsecured notes 2,213,811 1,725,662
Secured debt 817,834 794,710
Debt instrument 3,031,645 2,520,372
Estimated Fair Value    
Liabilities:    
Senior unsecured notes 2,292,648 1,810,882
Secured debt 793,815 783,353
Debt instrument $ 3,086,463 $ 2,594,235
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Liabilities:    
Fair value measurement, number of debt securities | debt_security 2  
Significant Other Observable Inputs (Level 2)    
Liabilities:    
Fair value measurement, number of debt securities | debt_security 5