Summary of significant accounting policies: Annual depreciation rates (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Depreciation
Depreciation expense	$ 180,866,000	$ 178,302,000	$ 171,240,000
Electric plant
Construction rates for allowance of funds (as a percent)	4.73%	4.97%	4.93%
Restricted cash and investments
Guaranteed interest rate on deposit with Rural Utilities Service (as a percent)	5.00%	5.00%
Restricted cash and investments	$ 387,961,000	$ 365,585,000
Restricted cash and investments - long-term	134,690,000	118,390,000
Inventories
Fossil fuels inventories	104,086,000	98,789,000
Spare parts	$ 195,166,000	$ 172,060,000
Steam production
Depreciation
Annual depreciation rates (as a percent)	1.93%	1.86%	1.82%
Steam production | Minimum
Depreciation
Range of Useful Life	49 years
Steam production | Maximum
Depreciation
Range of Useful Life	65 years
Nuclear production
Depreciation
Annual depreciation rates (as a percent)	1.55%	1.53%	1.54%
Nuclear production | Minimum
Depreciation
Range of Useful Life	37 years
Nuclear production | Maximum
Depreciation
Range of Useful Life	60 years
Hydro production
Depreciation
Range of Useful Life	50 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
Other production
Depreciation
Annual depreciation rates (as a percent)	2.38%	2.56%	2.55%
Other production | Minimum
Depreciation
Range of Useful Life	27 years
Other production | Maximum
Depreciation
Range of Useful Life	33 years
Transmission
Depreciation
Range of Useful Life	36 years
Annual depreciation rates (as a percent)	2.75%	2.75%	2.75%
General | Minimum
Depreciation
Range of Useful Life	3 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
General | Maximum
Depreciation
Range of Useful Life	50 years
Annual depreciation rates (as a percent)	33.33%	33.33%	33.33%