Subsequent Events (Details) (Rocky Mountain, USD $)	2 Months Ended		1 Months Ended
	Feb. 28, 1997 item	Jan. 31, 1997 item	Jul. 31, 2012 Subsequent events item	Jul. 12, 2012 Subsequent events item
Subsequent Events
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
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				3
Number of investors, whose leases were terminated			1
Termination of leases as a percentage of the original lease transactions				69.00%
Termination costs			$ 17,200,000
Net benefit recognized upon termination of lease			31,900,000
Net benefit associated with the portion of terminated lease recognized to income			14,700,000
Decrease in the deposit on hydro facility transactions			94,500,000
Decrease in obligation under hydro facility transactions			$ 94,500,000