Summary of significant accounting policies: Nuclear decommissioning information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Summary of significant accounting policies:
Additional contribution to external trust funds	$ 0	$ 0
Diversified mix of equity under external trust fund (as a percent)	60.00%
Diversified mix of fixed income securities under external trust fund (as a percent)	40.00%
Unrestricted Long term investments associated with internal decommissioning fund	$ 63,326,000	59,080,000
Additional amount collected for nuclear decommissioning	$ 4,750,000	2,975,000
Earning rate assumed on decommissioning trust fund (as a percent)	6.00%
Earnings on fund assets	$ 40,380,000	$ 23,507,000
Average annualized return produced by the nuclear decommissioning trust fund (as a percent)	7.10%