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Summary of Significant Accounting Policies (Utility Plant Intangible Assets) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Property, Plant and Equipment, Gross $ 1,666,381 $ 1,524,422
Finite-Lived Intangible Assets, Gross 23,989 22,675
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets   436
Infrastructure [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Finite-Lived Intangible Assets, Gross $ 2,524  
Infrastructure [Member] | Adjustments for New Accounting Pronouncement [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Property, Plant and Equipment, Gross   (1,859)
Finite-Lived Intangible Assets, Gross   1,859
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment   (377)
Finite-Lived Intangible Assets, Accumulated Amortization   $ 377
Minimum [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Useful life of intangible assets 5 years  
Maximum [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Useful life of intangible assets 70 years