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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
M
Dec. 31, 2010
Dec. 31, 2009
Depreciable plant and equipment      
Total $ 1,807,569 $ 1,695,075  
Provision for depreciation expressed as a percentage of the aggregate depreciable asset balances 3.00% 2.80% 2.80%
Asset Retirement Obligation      
Estimated retirement obligation 14,049 10,582  
Cash Equivalents      
Maximum original maturity period of cash equivalents (in months) 3    
Minimum
     
Depreciable plant and equipment      
Equipment, Useful Lives (in years) P5Y    
Transmission and distribution plant, Useful Lives (in years) P40Y    
Maximum
     
Depreciable plant and equipment      
Equipment, Useful Lives (in years) P50Y    
Transmission and distribution plant, Useful Lives (in years) P65Y    
Office Buildings and other structures, Useful Lives (in years) P50Y    
Utility Plant
     
Depreciable plant and equipment      
Equipment 382,195 321,958  
Transmission and distribution plant 1,287,010 1,263,895  
Office buildings and other structures 138,364 109,222  
Total $ 1,807,569 $ 1,695,075