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Indebtedness (Credit Facility) (Details)
12 Months Ended
Dec. 31, 2016
USD ($)
extension
Dec. 31, 2015
USD ($)
Line of Credit Facility [Line Items]    
Amount outstanding $ 0 $ 259,000,000
Letter of credit obligations $ 6,900,000  
Line of Credit [Member]    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.05%  
Commitment fee percentage 0.20%  
Expiration date Mar. 01, 2018  
Number of maturity extensions | extension 2  
Length of maturity extension 6 months  
Competitive bid percentage 50.00%  
Letter of credit obligations $ 6,900,000  
Maximum borrowing capacity 800,000,000  
Accordion feature borrowing capacity 400,000,000  
Borrowed funds for covenant ratio $ 0.60  
Covenant ratio 1.50  
Debt covenant percentage 60.00%  
Unencumbered net operating income to unsecured interest expense value for debt covenant 175.00%