Investments: (Details 3) (USD $)	2 Months Ended	12 Months Ended
	Jan. 31, 1997 item	Dec. 31, 2012 item
Rocky Mountain Lease Transaction
Net benefit associated with the portion of terminated lease recognized to income		$ 18,976,000
Rocky Mountain
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
Term of lease as a percentage of the estimated useful life of the jointly owned utility plant	120.00%
Term of lease	30 years
Number of leases terminated prior to end of lease term		5
Number of investors, whose leases were terminated		2
Termination of leases as a percentage of the original lease transactions		90.00%
Number of leases which remained in place		1
Percentage of leases which remained in place		10.00%
Termination costs		22,500,000
Net benefit recognized upon termination of lease		41,400,000
Net benefit associated with the portion of terminated lease recognized to income		18,900,000
Basic rental payments due		$ 70,500,000