Summary Of Significant Accounting Policies - Additional Information (Details)		3 Months Ended	12 Months Ended
	Dec. 13, 2016 USD ($)	Dec. 31, 2016 USD ($) member_class member representative	Dec. 31, 2016 USD ($) member_class member representative product segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Consolidated assets		$ 5,700,000	$ 5,700,000	$ 5,700,000
Number of classes of members | member_class		2	2
Number of Class A members | member		11	11
Number of representatives from each Class A member on the board of directors | representative		2	2
Number of representatives from each Class B member on the board of directors | representative		1	1
Frequency of depreciation study			5 years
Reimbursement of nuclear fuel costs receivable		$ 3,300,000	$ 3,300,000	4,300,000
Interest costs capitalized			30,300,000	13,800,000	$ 900,000
Non-member energy sales			$ 30,777,000	$ 51,103,000	$ 43,543,000
Number of power products for sale | product			2
Percentage of actual total interest charges			29.70%	20.00%	19.50%
Additional equity contribution	$ 5,800,000	5,800,000	$ 5,800,000
Adjustments under margin stabilization			15,100,000	$ 9,600,000	$ 0
Deferred energy, liability		40,000,000	40,000,000	27,800,000
Retirement of patronage capital	$ 5,800,000		5,756,000
Payment date of patronage capital	Apr. 03, 2017
Reduction of patronage capital	$ 5,800,000
Increase in accounts payable-members	$ 5,800,000
Accounts receivable–members		85,116,000	$ 85,116,000	83,248,000
Number of operating segments | segment			1
Number of reportable segments | segment			1
Debt issuance costs		6,400,000	$ 6,400,000	6,800,000
Long-term Debt [Member]
Significant Accounting Policies [Line Items]
Capitalized costs associated with the issuance of debt		6,400,000	6,400,000	6,800,000
Deferred Charges - Other [Member]
Significant Accounting Policies [Line Items]
Capitalized costs associated with the issuance of debt		$ 400,000	$ 400,000	700,000
Minimum [Member]
Significant Accounting Policies [Line Items]
Percentage of actual total interest charges			10.00%
Maximum [Member]
Significant Accounting Policies [Line Items]
Percentage of actual total interest charges			20.00%
Derivative term			3 years
TEC [Member]
Significant Accounting Policies [Line Items]
Percentage of interest owned in subsidiary by our Class A members			100.00%
Non-member energy sales			$ 0	$ 0	$ 0