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Regulatory assets and liabilities (Details) (USD $)
Dec. 31, 2013
Dec. 31, 2012
Regulatory assets and liabilities    
Regulatory assets: $ 305,373,000 $ 351,194,000
Regulatory liabilities: 432,325,000 450,482,000
Net regulatory position (126,952,000) (99,288,000)
Regulatory assets not earning a rate of return 163,700,000 215,600,000
Plant removal and decommissioning costs (c)
    
Regulatory assets and liabilities    
Regulatory liabilities: 308,431,000 [1] 296,037,000 [1]
Deferred income taxes
    
Regulatory assets and liabilities    
Regulatory liabilities: 64,914,000 [2] 82,077,000 [2]
Taxes refundable to customers (c)
    
Regulatory assets and liabilities    
Regulatory liabilities: 20,180,000 [1] 24,212,000 [1]
Natural gas costs refundable through rate adjustments (d)
    
Regulatory assets and liabilities    
Regulatory liabilities: 16,932,000 [3] 35,328,000 [3]
Other (c) (d)
    
Regulatory assets and liabilities    
Regulatory liabilities: 21,868,000 [1],[3] 12,828,000 [1],[3]
Deferred income taxes
    
Regulatory assets and liabilities    
Estimated recovery period ** [2]  
Regulatory assets: 125,607,000 121,781,000
Pension and postretirement benefits (a)
    
Regulatory assets and liabilities    
Estimated recovery period (e) [4]  
Regulatory assets: 105,123,000 [5] 166,477,000 [5]
Taxes recoverable from customers (a)
    
Regulatory assets and liabilities    
Estimated recovery period Over plant lives [6]  
Regulatory assets: 18,266,000 [5] 9,078,000 [5]
Manufactured gas plant sites remediation (a)
    
Regulatory assets and liabilities    
Estimated recovery period Up to 4 years [6]  
Regulatory assets: 15,797,000 [5] 15,828,000 [5]
Natural gas costs recoverable through rate adjustments (b)
    
Regulatory assets and liabilities    
Estimated recovery period Up to 28 months [6]  
Regulatory assets: 12,060,000 [7] 2,981,000 [7]
Long-term debt refinancing costs (a)
    
Regulatory assets and liabilities    
Estimated recovery period Up to 25 years [6]  
Regulatory assets: 8,697,000 [5] 9,144,000 [5]
Costs related to identifying generation development (a)
    
Regulatory assets and liabilities    
Estimated recovery period Up to 13 years [6]  
Regulatory assets: 4,512,000 [5] 5,773,000 [5]
Other (a) (b)
    
Regulatory assets and liabilities    
Estimated recovery period Largely within 1- 5 years [6]  
Regulatory assets: $ 15,311,000 [5],[7] $ 20,132,000 [5],[7]
[1] Included in other liabilities on the Consolidated Balance Sheets.
[2] Represents deferred income taxes related to regulatory assets and liabilities. The deferred income tax assets are not earning a rate of return.
[3] Included in other accrued liabilities on the Consolidated Balance Sheets.
[4] Recovered as expense is incurred.
[5] Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
[6] Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.
[7] Included in prepayments and other current assets on the Consolidated Balance Sheets.