Derivative instruments: Realized and unrealized gains and (losses) on derivative instruments (Details) - Not designated as hedges - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total gains (losses) on derivatives	$ (19,896)	$ 848	$ (3,312)
Unrealized gains and (losses) on derivatives	(48,763)	(68,146)
Natural gas swaps | Regulatory Asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(22,848)	(18,914)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	206	1,881	739
Losses	(20,102)	(1,033)	$ (4,051)
Interest rate options | Regulatory Asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	$ (25,915)	$ (49,232)