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Summary of Significant Accounting Policies (Details 5) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2011 | Dec. 31, 2010
Revenue recognition [Abstract] |  |
Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain | $ 80.2 | $ 87.3
Percentage-of-completion method [Abstract] |  |
Costs and estimated earnings in excess of billings on uncompleted contracts | 54.3 | 46.6
Billings in excess of costs and estimated earnings on uncompleted contracts | 79.1 | 65.2
Amounts representing balances billed but not paid by customers under retainage provisions in contracts | 51.5 | 51.1
Amounts of receivable retainage expected to be paid within one year or less | 49.3 | 50.4
Long-term receivable retainage | 2.2 | 0.7
Derivative instruments [Abstract] |  |
Maximum period allowed to hedge monthly forecasted sales of natural gas and oil production at Fidelity (in months) | 36 months |
Maximum period allowed to hedge interest rate derivative instruments (in months) | 24 months |
Maximum period allowed to hedge foreign currency derivative instruments (in months) | 12-month period |
Maximum period allowed to hedge monthly forecasted purchases of natural gas at Cascade and Intermountain (in years) | three years |
Maximum period for settlement of all interest rate derivative transactions (in days) | 90 days |
Maximum period for settlement of foreign currency derivative transaction (in months) | 12-month period |
Natural gas costs recoverable or refundable through rate adjustments [Abstract] |  |
Minimum period over which natural gas costs greater or less than amounts being recovered become recoverable or refundable through rate adjustments (in months) | 12 |
Maximum period over which natural gas costs greater or less than amounts being recovered become recoverable or refundable through rate adjustments (in months) | 28 months |
Natural gas costs refundable through rate adjustments | 45.1 | 37.0
Natural gas costs recoverable through rate adjustments | 2.6 | 6.6
Insurance [Abstract] |  |
Maximum amount of deductibles for workers' compensation insurance per occurrence | 1 |
Maximum amount of deductibles for automobile liability and general liability insurance per accident or occurrence | $ 1 |
Income taxes [Abstract] |  |
Threshold of likelihood of tax benefits being realized upon ultimate settlement with a taxing authority (in hundredths) | 50.00% |

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