Pension Plan and Other Postretirement Benefits - Components of net periodic pension benefit cost (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost - Benefit Earned During the Period		$ 1,376	$ 1,514	$ 4,139	$ 4,544
Interest Cost on Projected Benefit Obligation		3,603	3,336	10,646	10,008
Expected Return on Assets		(4,857)	(4,595)	(14,590)	(13,787)
Amortization of Prior-Service Cost:
From Regulatory Asset		48	47	142	141
From Other Comprehensive Income	[1]	1	2	3	4
Amortization of Net Actuarial Loss:
From Regulatory Asset		1,411	1,669	3,865	5,007
From Other Comprehensive Income	[1]	32	42	95	128
Net Periodic Cost		1,614	2,015	4,300	6,045
Executive Survivor and Supplemental Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost - Benefit Earned During the Period		63	48	189	142
Interest Cost on Projected Benefit Obligation		417	380	1,251	1,142
Amortization of Prior-Service Cost:
From Regulatory Asset		4	5	12	13
From Other Comprehensive Income	[2]	9	9	28	28
Amortization of Net Actuarial Loss:
From Regulatory Asset		74	83	220	250
From Other Comprehensive Income	[3]	111	151	334	452
Net Periodic Cost		678	676	2,034	2,027
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost - Benefit Earned During the Period		365	324	976	972
Interest Cost on Projected Benefit Obligation		794	524	1,877	1,573
Amortization of Prior-Service Cost:
From Regulatory Asset		34	52	100	154
From Other Comprehensive Income	[1]	1	1	3	4
Amortization of Net Actuarial Loss:
From Regulatory Asset		284		284
From Other Comprehensive Income	[1]	7		7
Net Periodic Cost		1,485	901	3,247	2,703
Effect of Medicare Part D Subsidy		$ (177)	$ (372)	$ (692)	$ (1,115)

[1] Corporate cost included in Other Nonelectric Expenses.

[2] 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

[3] 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