Pension Plan and Other Postretirement Benefits - Components of net periodic pension benefit cost (Parentheticals) (Details) - Executive Survivor and Supplemental Retirement Plan - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Amortization of Prior Service Costs from Other Comprehensive Income Charged to:	[1]	$ 9	$ 9	$ 28	$ 28
2Amortization of Net Actuarial Loss from Other Comprehensive Income Charged to:	[2]	111	151	334	452
Electric operation and maintenance expenses
Defined Benefit Plan Disclosure [Line Items]
Amortization of Prior Service Costs from Other Comprehensive Income Charged to:		3	3	11	11
2Amortization of Net Actuarial Loss from Other Comprehensive Income Charged to:		68	78	204	233
Other nonelectric expenses
Defined Benefit Plan Disclosure [Line Items]
Amortization of Prior Service Costs from Other Comprehensive Income Charged to:		6	6	17	17
2Amortization of Net Actuarial Loss from Other Comprehensive Income Charged to:		$ 43	$ 73	$ 130	$ 219

[1] Amortization of Prior Service Costs from Other Comprehensive Income Charged to: Electric Operation and Maintenance Expenses $ 3 $ 3 $ 11 $ 11 Other Nonelectric Expenses 6 6 17 17

[2] Amortization of Net Actuarial Loss from Other Comprehensive Income Charged to: Electric Operation and Maintenance Expenses $ 68 $ 78 $ 204 $ 233 Other Nonelectric Expenses 43 73 130 219