Investments: Rocky Mountain information (Details) - Rocky Mountain	2 Months Ended	12 Months Ended
	Jan. 31, 1997 item	Dec. 31, 2012 USD ($) 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
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
Percentage of leases which remained in place		10.00%
Basic rental payments due | $		$ 59,209,000