Summary of significant accounting policies: - Nuclear decommissioning information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Nuclear decommissioning
Additional contribution to external trust funds	$ 0	$ 0
Additional amount collected for nuclear decommissioning	$ 4,750,000	$ 4,750,000
Percentage of decommissioning fund classified as equity	60.00%	60.00%
Percentage of decommissioning funds classified as fixed income securities	40.00%	40.00%
External and Internal Trust Funds:
Cost	$ 442,148,000	$ 425,078,000
Average annualized return produced by the nuclear decommissioning trust fund (as a percent)	5.90%
External and Internal Trust Funds
External and Internal Trust Funds:
Net realized gains or losses, interest income and dividends, contributions and fees	$ 12,270,144	44,871,375
External Trust Funds
External and Internal Trust Funds:
Cost		342,639,000	$ 314,258,000
Purchases	395,506,000	558,568,000
Net Proceeds	(389,189,000)	(530,186,000)
Unrealized Gain (Loss)	37,073,000	21,189,000
Fair Value	386,029,000	363,829,000
Internal Trust Funds
External and Internal Trust Funds:
Cost		59,052,000	42,563,000
Purchases	42,783,000	82,554,000
Net Proceeds	(36,830,000)	(66,065,000)
Unrealized Gain (Loss)	7,052,000	4,274,000
Fair Value	72,057,000	63,326,000
Equity
External and Internal Trust Funds:
Cost	237,317,000	230,123,000
Equity | External Trust Funds
External and Internal Trust Funds:
Cost		198,265,000	183,527,000
Purchases	46,865,000	104,417,000
Net Proceeds	(43,395,000)	(77,504,000)
Unrealized Gain (Loss)	38,749,000	23,321,000
Fair Value	240,484,000	233,761,000
Equity | Internal Trust Funds
External and Internal Trust Funds:
Cost		33,513,000	19,796,000
Purchases	0	0
Net Proceeds	5,285,000	13,717,000
Unrealized Gain (Loss)	7,263,000	4,537,000
Fair Value	46,061,000	38,050,000
Debt
External and Internal Trust Funds:
Cost	201,492,000	189,700,000
Debt | External Trust Funds
External and Internal Trust Funds:
Cost		144,187,000	130,407,000
Purchases	347,383,000	454,072,000
Net Proceeds	(343,040,000)	(450,747,000)
Unrealized Gain (Loss)	(1,675,000)	(2,128,000)
Fair Value	146,855,000	131,604,000
Debt | Internal Trust Funds
External and Internal Trust Funds:
Cost		25,539,000	22,767,000
Purchases	42,783,000	82,554,000
Net Proceeds	(42,115,000)	(79,782,000)
Unrealized Gain (Loss)	(211,000)	(263,000)
Fair Value	25,996,000	25,276,000
Other | External Trust Funds
External and Internal Trust Funds:
Cost		187,000	$ 324,000
Purchases	1,258,000	79,000
Net Proceeds	(2,754,000)	(1,935,000)
Unrealized Gain (Loss)	(1,000)	(4,000)
Fair Value	$ (1,310,000)	$ (1,536,000)