Summary of significant accounting policies: - Depreciation and related information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Depreciation
Depreciation expense	$ 211,282,000	$ 180,866,000	$ 178,302,000
Steam production
Depreciation
Annual depreciation rates (as a percent)	2.84%	1.93%	1.86%
Steam production | Minimum
Depreciation
Useful Life	49 years
Steam production | Maximum
Depreciation
Useful Life	65 years
Nuclear production
Depreciation
Annual depreciation rates (as a percent)	1.96%	1.55%	1.53%
Nuclear production | Minimum
Depreciation
Useful Life	37 years
Nuclear production | Maximum
Depreciation
Useful Life	60 years
Hydro production
Depreciation
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)	2.55%	2.38%	2.56%
Other production | Minimum
Depreciation
Useful Life	27 years
Other production | Maximum
Depreciation
Useful Life	33 years
Transmission
Depreciation
Useful Life	36 years
Annual depreciation rates (as a percent)	2.75%	2.75%	2.75%
General | Minimum
Depreciation
Useful Life	3 years
Annual depreciation rates (as a percent)	2.00%	2.00%	2.00%
General | Maximum
Depreciation
Useful Life	50 years
Annual depreciation rates (as a percent)	33.33%	33.33%	33.33%