Rocky Mountain Lease Transactions (Details) (USD $)	3 Months Ended	9 Months Ended	1 Months Ended		2 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Oct. 31, 2012 Rocky Mountain item	Jul. 31, 2012 Rocky Mountain item	Feb. 28, 1997 Rocky Mountain item	Jan. 31, 1997 Rocky Mountain item	Sep. 30, 2012 Rocky Mountain item	Sep. 30, 2012 Rocky Mountain	Oct. 18, 2012 Rocky Mountain item	Jul. 12, 2012 Rocky Mountain item
Rocky Mountain Lease Transaction
Number of long-term lease transactions						6
Percentage of undivided ownership interest						74.61%
Number of separate owner trusts to whom undivided interest was leased					6
Number of investors in ownership trusts					3		2
Term of lease as a percentage of the estimated useful life of the jointly owned utility plant					120.00%
Number of leases terminated prior to end of lease term									2	3
Number of investors, whose leases were terminated			1	1
Termination of leases as a percentage of the original lease transactions									21.00%	69.00%
Termination costs			$ 5,300,000	$ 17,200,000
Net benefit recognized upon termination of lease			9,532,000	31,900,000
Net benefit associated with the portion of terminated lease recognized to income	14,719,000	14,719,000	4,232,000	14,700,000
Decrease in the deposit on hydro facility transactions			28,775,000
Decrease in obligation under hydro facility transactions								$ 94,500,000