Annual report pursuant to Section 13 and 15(d)

Debt, Net (Details 3)

v3.8.0.1
Debt, Net (Details 3) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Carrying Amount [Member]    
Debt Instrument [Line Items]    
Long-term debt $ 1,828,333 $ 1,904,001
Carrying Amount [Member] | Unsecured Senior Notes [Member]    
Debt Instrument [Line Items]    
Long-term debt 1,189,249 1,187,515
Carrying Amount [Member] | Other fixed-rate debt [Member]    
Debt Instrument [Line Items]    
Long-term debt 152,010 155,544
Carrying Amount [Member] | Variable-rate debt [Member]    
Debt Instrument [Line Items]    
Long-term debt 487,074 560,942
Estimated Fair Value [Member]    
Debt Instrument [Line Items]    
Long-term debt 1,867,577 1,935,606
Estimated Fair Value [Member] | Unsecured Senior Notes [Member]    
Debt Instrument [Line Items]    
Long-term debt 1,229,398 1,220,282
Estimated Fair Value [Member] | Other fixed-rate debt [Member]    
Debt Instrument [Line Items]    
Long-term debt 152,485 156,887
Estimated Fair Value [Member] | Variable-rate debt [Member]    
Debt Instrument [Line Items]    
Long-term debt $ 485,694 $ 558,437