Quarterly report pursuant to Section 13 or 15(d)

Debt, Net (Details 2)

v3.8.0.1
Debt, Net (Details 2) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Carrying Amount    
Carrying amount and estimated fair value of debt    
Long-term debt $ 1,854,886 $ 1,828,333
Carrying Amount | Unsecured senior notes    
Carrying amount and estimated fair value of debt    
Long-term debt 1,189,692 1,189,249
Carrying Amount | Other fixed-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 151,093 152,010
Carrying Amount | Variable-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 514,101 487,074
Estimated Fair Value    
Carrying amount and estimated fair value of debt    
Long-term debt 1,885,744 1,867,577
Estimated Fair Value | Unsecured senior notes    
Carrying amount and estimated fair value of debt    
Long-term debt 1,222,102 1,229,398
Estimated Fair Value | Other fixed-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 151,271 152,485
Estimated Fair Value | Variable-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt $ 512,371 $ 485,694