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%