Quarterly report pursuant to Section 13 or 15(d)

Debt, Net (Details 2)

v3.7.0.1
Debt, Net (Details 2) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Carrying Amount    
Carrying amount and estimated fair value of debt    
Long-term debt $ 1,897,734 $ 1,904,001
Carrying Amount | Unsecured senior notes    
Carrying amount and estimated fair value of debt    
Long-term debt 1,188,373 1,187,515
Carrying Amount | Other fixed-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 153,795 155,544
Carrying Amount | Variable-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 555,566 560,942
Estimated Fair Value    
Carrying amount and estimated fair value of debt    
Long-term debt 1,937,717 1,935,606
Estimated Fair Value | Unsecured senior notes    
Carrying amount and estimated fair value of debt    
Long-term debt 1,229,354 1,220,282
Estimated Fair Value | Other fixed-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt 155,581 156,887
Estimated Fair Value | Variable-rate debt    
Carrying amount and estimated fair value of debt    
Long-term debt $ 552,782 $ 558,437