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Commitments and Contingencies (Details 2) (USD $)
 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2013
Vehicular traffic accident
Sep. 30, 2013
L.E. Myers Co
Vehicular traffic accident
item
Jan. 31, 2013
L.E. Myers Co
Northern States Power Company (Wisconsin) v. The City of Ashland, Wisconsin
Minimum
Jan. 31, 2013
L.E. Myers Co
Northern States Power Company (Wisconsin) v. The City of Ashland, Wisconsin
Maximum
Dec. 31, 2013
Insurance and Claims Accruals
Dec. 31, 2012
Insurance and Claims Accruals
Dec. 31, 2011
Insurance and Claims Accruals
Dec. 31, 2013
Surety bonds
Commitments and Contingencies                    
Deductible per claim for each line of coverage, until claim aggregate is reached $ 1,000,000                  
Deductible under health insurance benefits for qualified individuals 100,000                  
Aggregate original face amount of outstanding bonds issued by the surety                   889,100,000
Estimated remaining cost to complete bonded projects                   147,600,000
Roll forward of activity in the insurance reserves                    
Balance at beginning of period 39,583,000 38,850,000                
Net increases in reserves 16,861,000 19,743,000                
Net payments made (17,333,000) (19,010,000)                
Balance at end of period 39,111,000 39,583,000                
Insurance expense             20,100,000 20,900,000 21,500,000  
Compensatory damages       700,000            
Punitive damages       3,600,000            
Maximum number of times of compensatory damages for estimated punitive damages       3            
Reserve for legal settlements     2,300,000              
Period during which the defendant operated a manufactured gas plant         6 years 12 years        
Damages alleged           $ 140,000,000