Bradley Lake Hydroelectric Project (Schedule Of Bradley Lake Information) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-term Contract for Purchase of Electric Power [Line Items]
Plant in service	$ 1,223,044,797				$ 1,210,969,809				$ 1,223,044,797		$ 1,210,969,809
Long-term debt	485,605,998								485,605,998
Interest expense	$ 5,575,665	$ 5,569,961	$ 5,535,031	$ 5,520,479	$ 5,341,242	$ 5,427,440	$ 5,247,404	$ 5,385,211	$ 22,201,136		$ 21,401,297	$ 21,814,445
Bradley Lake Hydroelectric Project [Member] | Alaska Energy Authority [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Plant in service			162,907,000							$ 162,907,000
Long-term debt			43,940,000							43,940,000
Interest expense										2,652,000
Bradley Lake Hydroelectric Project [Member] | Alaska Energy Authority [Member] | Proportionate Share [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Plant in service			49,524,000							49,524,000
Long-term debt			$ 13,358,000							13,358,000
Interest expense										$ 806,000