Derivative instruments: - Realized and unrealized gains and (losses) on derivative instruments (Details) - Not designated as hedges - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Total gains (losses) on derivatives	$ (17,252)	$ (19,896)	$ 848
Total unrealized gains (losses)	9,302	(48,763)
Natural gas swaps | Regulatory liability
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized gains on derivatives	15,152
Natural gas swaps | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	(62)	(22,848)
Natural gas swaps | Fuel
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Gains	2,445	206	1,881
Losses	(19,697)	(20,102)	$ (1,033)
Interest rate options | Regulatory asset
Effect of Derivative Instruments on the Condensed Statement of Revenues and Expenses or Balance Sheet
Unrealized losses on derivatives	$ (5,788)	$ (25,915)