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Commitments (Commitments) (Narrative) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
d
Rate
Dec. 31, 2016
USD ($)
Unsecured debt [Member] | Line of credit facility [Member]    
Commitments    
Letters of credit | $ $ 36.8 $ 38.7
Senior notes [Member]    
Commitments    
Debt instrument minimum number of days callable | d 30  
Debt instrument maximum number of days callable | d 60  
Percentage principal amount of debt that can be redeemed by the Company | Rate 100.00%  
Percentage principal amount of debt redeemable upon change in control | Rate 101.00%  
Construction [Member]    
Commitments    
Construction commitments | $ $ 54.4