Quarterly report pursuant to Section 13 or 15(d)

Debt, Net (Details 2)

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Debt, Net (Details 2) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Carrying Amount    
Carrying amount and estimated fair value of debt    
Long-term debt $ 1,871,445 $ 1,828,333
Carrying Amount | Unsecured senior notes    
Carrying amount and estimated fair value of debt    
Long-term debt 1,190,139 1,189,249
Carrying Amount | Other fixed-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 150,178 152,010
Carrying Amount | Variable-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 531,128 487,074
Estimated Fair Value    
Carrying amount and estimated fair value of debt    
Long-term debt 1,899,265 1,867,577
Estimated Fair Value | Unsecured senior notes    
Carrying amount and estimated fair value of debt    
Long-term debt 1,220,040 1,229,398
Estimated Fair Value | Other fixed-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 148,257 152,485
Estimated Fair Value | Variable-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt $ 530,968 $ 485,694