Pension Plan and Other Postretirement Benefits - Components of net periodic pension benefit cost (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost - Benefit Earned During the Period	$ 1,418		$ 1,248		$ 2,836		$ 2,542
Interest Cost on Projected Benefit Obligation	3,036		3,125		6,072		6,233
Expected Return on Assets	(3,632)		(3,607)		(7,264)		(7,215)
Amortization of Prior-Service Cost:
From Regulatory Asset	83		100		166		199
From Other Comprehensive Income	2	[1]	2	[1]	4	[1]	5	[1]
Amortization of Net Actuarial Loss:
From Regulatory Asset	1,663		1,255		3,326		2,454
From Other Comprehensive Income	45	[1]	34	[1]	90	[1]	66	[1]
Net Periodic Pension Cost	2,615		2,157		5,230		4,284
Executive Survivor and Supplemental Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost - Benefit Earned During the Period	13		12		26		23
Interest Cost on Projected Benefit Obligation	352		369		704		739
Amortization of Prior-Service Cost:
From Regulatory Asset	5		6		10		11
From Other Comprehensive Income	13	[2]	13	[2]	26	[2]	26	[2]
Amortization of Net Actuarial Loss:
From Regulatory Asset	52		38		104		77
From Other Comprehensive Income	78	[3]	43	[3]	156	[3]	86	[3]
Net Periodic Pension Cost	513		481		1,026		962
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost - Benefit Earned During the Period	441		374		882		772
Interest Cost on Projected Benefit Obligation	610		630		1,220		1,287
Amortization of Transition Obligation:
From Regulatory Asset			182				364
From Other Comprehensive Income		[1]	5	[1]		[1]	10	[1]
Amortization of Prior-Service Cost:
From Regulatory Asset	51		52		102		103
From Other Comprehensive Income	1	[1]	1	[1]	2	[1]	2	[1]
Amortization of Net Actuarial Loss:
From Regulatory Asset	248		135		496		321
From Other Comprehensive Income	6	[1]	4	[1]	12	[1]	9	[1]
Net Periodic Pension Cost	1,357		1,383		2,714		2,868
Effect of Medicare Part D Subsidy	$ (564)		$ (533)		$ (1,128)		$ (1,020)

[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 $ 5 $ 5 $ 10 $ 10 Other Nonelectric Expenses 8 8 16 16

[3] Amortization of Net Actuarial Loss from Other Comprehensive Income Charged to: Electric Operation and Maintenance Expenses $ 48 $ 36 $ 96 $ 72 Other Nonelectric Expenses 30 7 60 14